My tasks error out with this message:
OpenCL platform detected: Intel
OpenCL platform detected: Mesa
OpenCL platform detected: The pocl project
WARNING: BOINC supplied wrong platform!

Boinc Manager messages upon start:
Starting BOINC client version 7.9.3 for x86_64-pc-linux-gnu
Libraries: libcurl/7.58.0 OpenSSL/1.1.0g zlib/1.2.11 libidn2/2.0.4 libpsl/0.19.1 (+libidn2/2.0.4) nghttp2/1.30.0 librtmp/2.3
Data directory: /var/lib/boinc-client
OpenCL: Intel GPU 0: Intel HD Graphics Family (driver version 1.3, device version OpenCL 2.0 beignet 1.3, 4096MB, 4096MB available, 144 GFLOPS peak)
[libc detection] gathered: 2.27, Ubuntu GLIBC 2.27-3ubuntu1
Processor: 4 GenuineIntel Intel(R) Pentium(R) Silver J5005 CPU @ 1.50GHz [Family 6 Model 122 Stepping 1]
OS: Linux LinuxMint: Linux Mint 19.1 Tessa [4.15.0-47-generic|libc 2.27 (Ubuntu GLIBC 2.27-3ubuntu1)]
Memory: 15.32 GB physical, 15.43 GB virtual
VirtualBox version: 5.2.18_Ubuntur123745

Should I remove pocl?

clinfo:
Number of platforms 3
Platform Name Intel Gen OCL Driver
Platform Vendor Intel
Platform Version OpenCL 2.0 beignet 1.3
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_global_int32_base_atomics cl_khr_global_int32_extended_atomics cl_khr_local_int32_base_atomics cl_khr_local_int32_extended_atomics cl_khr_byte_addressable_store cl_khr_3d_image_writes cl_khr_image2d_from_buffer cl_khr_depth_images cl_khr_spir cl_khr_icd cl_intel_accelerator cl_intel_subgroups cl_intel_subgroups_short cl_khr_gl_sharing
Platform Extensions function suffix Intel

Platform Name Portable Computing Language
Platform Vendor The pocl project
Platform Version OpenCL 1.2 pocl 1.1 None+Asserts, LLVM 6.0.0, SPIR, SLEEF, DISTRO, POCL_DEBUG
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_icd
Platform Extensions function suffix POCL

Platform Name Clover
Platform Vendor Mesa
Platform Version OpenCL 1.1 Mesa 18.2.8
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_icd
Platform Extensions function suffix MESA

OpenCL platform detected: Intel
OpenCL platform detected: Mesa
OpenCL platform detected: The pocl project
WARNING: BOINC supplied wrong platform!
ERROR: OpenCL kernel/call 'clGetDeviceIDs' call failed (-1) in file ../../src/GPU_lock.cpp near line 1122.

Installed Intel OpenCL SDK for Linux, which resulted in a slightly other error message:

OpenCL platform detected: Mesa
OpenCL platform detected: The pocl project
OpenCL platform detected: Intel(R) Corporation
WARNING: BOINC supplied wrong platform!
ERROR: OpenCL kernel/call 'clGetDeviceIDs' call failed (-1) in file ../../src/GPU_lock.cpp near line 1122.

Starting BOINC client version 7.9.3 for x86_64-pc-linux-gnu
log flags: file_xfer, sched_ops, task
Libraries: libcurl/7.58.0 OpenSSL/1.1.0g zlib/1.2.11 libidn2/2.0.4 libpsl/0.19.1 (+libidn2/2.0.4) nghttp2/1.30.0 librtmp/2.3
Data directory: /var/lib/boinc-client
OpenCL: Intel GPU 0: Intel HD Graphics Family (driver version 1.3, device version OpenCL 2.0 beignet 1.3, 4096MB, 4096MB available, 144 GFLOPS peak)
OpenCL CPU: Intel(R) Pentium(R) Silver J5005 CPU @ 1.50GHz (OpenCL driver vendor: Intel(R) Corporation, driver version 18.1.0.0920, device version OpenCL 2.1 (Build 0))
[libc detection] gathered: 2.27, Ubuntu GLIBC 2.27-3ubuntu1
Host name: Asrock-J5005-iTX
Processor: 4 GenuineIntel Intel(R) Pentium(R) Silver J5005 CPU @ 1.50GHz [Family 6 Model 122 Stepping 1]
Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg cx16 xtpr pdcm sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave rdrand lahf_lm 3dnowprefetch cpuid_fault cat_l2 pti cdp_l2 ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust smep erms mpx rdt_a rdseed smap clflushopt intel_pt sha_ni xsaveopt xsavec xgetbv1 xsaves dtherm ida arat pln pts umip rdpid arch_capabilities
OS: Linux LinuxMint: Linux Mint 19.1 Tessa [4.15.0-47-generic|libc 2.27 (Ubuntu GLIBC 2.27-3ubuntu1)]
Memory: 15.32 GB physical, 15.43 GB virtual
Disk: 314.28 GB total, 192.06 GB free
Local time is UTC +2 hours
VirtualBox version: 5.2.18_Ubuntur123745

Clinfo
Number of platforms 4
Platform Name Clover
Platform Vendor Mesa
Platform Version OpenCL 1.1 Mesa 18.2.8
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_icd
Platform Extensions function suffix MESA

Platform Name Portable Computing Language
Platform Vendor The pocl project
Platform Version OpenCL 1.2 pocl 1.1 None+Asserts, LLVM 6.0.0, SPIR, SLEEF, DISTRO, POCL_DEBUG
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_icd
Platform Extensions function suffix POCL

Platform Name Intel(R) CPU Runtime for OpenCL(TM) Applications
Platform Vendor Intel(R) Corporation
Platform Version OpenCL 2.1 LINUX
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_icd cl_khr_global_int32_base_atomics cl_khr_global_int32_extended_atomics cl_khr_local_int32_base_atomics cl_khr_local_int32_extended_atomics cl_khr_byte_addressable_store cl_khr_depth_images cl_khr_3d_image_writes cl_intel_exec_by_local_thread cl_khr_spir cl_khr_fp64 cl_khr_image2d_from_buffer cl_intel_vec_len_hint
Platform Host timer resolution 1ns
Platform Extensions function suffix INTEL

Platform Name Intel Gen OCL Driver
Platform Vendor Intel
Platform Version OpenCL 2.0 beignet 1.3
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_global_int32_base_atomics cl_khr_global_int32_extended_atomics cl_khr_local_int32_base_atomics cl_khr_local_int32_extended_atomics cl_khr_byte_addressable_store cl_khr_3d_image_writes cl_khr_image2d_from_buffer cl_khr_depth_images cl_khr_spir cl_khr_icd cl_intel_accelerator cl_intel_subgroups cl_intel_subgroups_short cl_khr_gl_sharing
Platform Extensions function suffix Intel

Removing Beignet results in 'No useable GPU detected'....

So what have we established so far?

out-of-the-box Linux -at least Linux Mint 19.1, which is built on Ubuntu 18.04- does not provide an OpenCL capable GPU for boinc, despite the log message

OpenCL: Intel GPU 0: Intel HD Graphics Family (driver version 1.3, device version OpenCL 2.0 beignet 1.3, 4096MB, 4096MB available, 144 GFLOPS peak)

The GPU gets detected by both OS and Boinc-client but tasks fail with

OpenCL platform detected: Intel
OpenCL platform detected: Mesa
OpenCL platform detected: The pocl project
WARNING: BOINC supplied wrong platform!
ERROR: OpenCL kernel/call 'clGetDeviceIDs' call failed (-1) in file ../../src/GPU_lock.cpp near line 1122.

Installing the Intel OpenCL SDK for Linux (free subscription required) resuts in an OpenCL capable CPU -not GPU- and gives

OpenCL: Intel GPU 0: Intel HD Graphics Family (driver version 1.3, device version OpenCL 2.0 beignet 1.3, 4096MB, 4096MB available, 144 GFLOPS peak)
OpenCL CPU: Intel(R) Pentium(R) Silver J5005 CPU @ 1.50GHz (OpenCL driver vendor: Intel(R) Corporation, driver version 18.1.0.0920, device version OpenCL 2.1 (Build 0))

OpenCL platform detected: Mesa
OpenCL platform detected: The pocl project
OpenCL platform detected: Intel(R) Corporation
WARNING: BOINC supplied wrong platform!
ERROR: OpenCL kernel/call 'clGetDeviceIDs' call failed (-1) in file ../../src/GPU_lock.cpp near line 1122.

Removing Beignet results in 'No useable GPU detected', so Beignet is the culprit
Via Github I downloaded and installed a replacement for Beignet: Neo, aka Intel Compute Runtime
The first task is now running...

By now my first Intel OpenCL WU has completed and validated
OpenCL platform detected: Intel(R) Corporation
Number of OpenCL devices found : 1
BOINC assigns slot on device #0.
Info: BOINC provided OpenCL device ID used

Boinc Manager startup shows this:
Starting BOINC client version 7.9.3 for x86_64-pc-linux-gnu
log flags: file_xfer, sched_ops, task
Libraries: libcurl/7.58.0 OpenSSL/1.1.0g zlib/1.2.11 libidn2/2.0.4 libpsl/0.19.1 (+libidn2/2.0.4) nghttp2/1.30.0 librtmp/2.3
Data directory: /var/lib/boinc-client
OpenCL: Intel GPU 0: Intel(R) Gen9 HD Graphics NEO (driver version 19.14.12751, device version OpenCL 1.2 NEO, 3277MB, 3277MB available, 115 GFLOPS peak)
OpenCL CPU: Intel(R) Pentium(R) Silver J5005 CPU @ 1.50GHz (OpenCL driver vendor: Intel(R) Corporation, driver version 18.1.0.0920, device version OpenCL 2.1 (Build 0))
[libc detection] gathered: 2.27, Ubuntu GLIBC 2.27-3ubuntu1
Host name: Asrock-J5005-iTX
Processor: 4 GenuineIntel Intel(R) Pentium(R) Silver J5005 CPU @ 1.50GHz [Family 6 Model 122 Stepping 1]
Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg cx16 xtpr pdcm sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave rdrand lahf_lm 3dnowprefetch cpuid_fault cat_l2 pti cdp_l2 ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust smep erms mpx rdt_a rdseed smap clflushopt intel_pt sha_ni xsaveopt xsavec xgetbv1 xsaves dtherm ida arat pln pts umip rdpid arch_capabilities
OS: Linux LinuxMint: Linux Mint 19.1 Tessa [4.15.0-47-generic|libc 2.27 (Ubuntu GLIBC 2.27-3ubuntu1)]
Memory: 15.32 GB physical, 15.43 GB virtual
Disk: 314.28 GB total, 192.12 GB free
Local time is UTC +2 hours
VirtualBox version: 5.2.18_Ubuntur123745

clinfo comes with
Number of platforms 4
Platform Name Intel(R) OpenCL HD Graphics
Platform Vendor Intel(R) Corporation
Platform Version OpenCL 1.2
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_3d_image_writes cl_khr_byte_addressable_store cl_khr_fp16 cl_khr_depth_images cl_khr_global_int32_base_atomics cl_khr_global_int32_extended_atomics cl_khr_icd cl_khr_image2d_from_buffer cl_khr_local_int32_base_atomics cl_khr_local_int32_extended_atomics cl_intel_subgroups cl_intel_required_subgroup_size cl_intel_subgroups_short cl_khr_spir cl_intel_accelerator cl_intel_media_block_io cl_intel_driver_diagnostics cl_intel_device_side_avc_motion_estimation cl_khr_priority_hints cl_khr_throttle_hints cl_khr_create_command_queue cl_khr_fp64 cl_intel_planar_yuv cl_intel_packed_yuv cl_intel_motion_estimation cl_intel_advanced_motion_estimation cl_intel_va_api_media_sharing
Platform Extensions function suffix INTEL

Platform Name Clover
Platform Vendor Mesa
Platform Version OpenCL 1.1 Mesa 18.2.8
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_icd
Platform Extensions function suffix MESA

Platform Name Portable Computing Language
Platform Vendor The pocl project
Platform Version OpenCL 1.2 pocl 1.1 None+Asserts, LLVM 6.0.0, SPIR, SLEEF, DISTRO, POCL_DEBUG
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_icd
Platform Extensions function suffix POCL

Platform Name Intel(R) CPU Runtime for OpenCL(TM) Applications
Platform Vendor Intel(R) Corporation
Platform Version OpenCL 2.1 LINUX
Platform Profile FULL_PROFILE
Platform Extensions cl_khr_icd cl_khr_global_int32_base_atomics cl_khr_global_int32_extended_atomics cl_khr_local_int32_base_atomics cl_khr_local_int32_extended_atomics cl_khr_byte_addressable_store cl_khr_depth_images cl_khr_3d_image_writes cl_intel_exec_by_local_thread cl_khr_spir cl_khr_fp64 cl_khr_image2d_from_buffer cl_intel_vec_len_hint
Platform Host timer resolution 1ns
Platform Extensions function suffix INTEL