I have recently installed Boinc on my Mac mini. Every task completed as a computational error. And I go no credit. Is there an issue with the software installation?
CPU
GenuineIntel Intel(R) Core(TM) i5-4308U CPU @ 2.80GHz [x86 Family 6 Model 69 Stepping 1] (4 processors)
GPU
INTEL Iris (1536MB) OpenCL: 1.2
OS Darwin 16.1.0

Doing a search about it and looking in the source code, this happens when BOINC checks if the task is finished, finds a finished file but it has been there for more than 10 seconds. It hasn't uploaded, it hasn't been reported, so it's aborted. This can happen when the client doesn't have enough time to clean up after itself, because other background processes take up too much of the CPU or when the disk is filled quite a lot.

Great, you think, so solution? Well, that's going to be difficult. There is none so easily said.
You could try BOINC 7.6.33 (which is now recommended), but its code has the same 10 seconds check for the finished file.

Perhaps that you set that BOINC should only upload, download & report work during a certain time group of the day, that that helps. (computing prefs, Transfer files only between.)

But I'd do a good clean up of the system. Get some more free space on the disk BOINC's data directory is on, and perhaps limit the amount of other background processes when BOINC runs.

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I'm getting this for an error on a large number of my work items. Is it the same issue as above?

<core_client_version>7.6.33</core_client_version>
<![CDATA[
<message>
process exited with code 225 (0xe1, -31)
</message>
<stderr_txt>
OpenCL platform detected: Apple
Number of OpenCL devices found : 1
BOINC assigns slot on device #0.
Info: BOINC provided OpenCL device ID used
WARNING: This application needs newer GPU, at least ATI Radeon HD 5000 needed, exiting !
</stderr_txt>
]]>

My grphcs card is an ATI HD4850

No, it's not the same as above. The problem is detailed in the error message that you posted yourself:
WARNING: This application needs newer GPU, at least ATI Radeon HD 5000 needed, exiting !

The minimum required AMD/ATI GPU for this project for the Mac is a HD 5000.
It may be weird you're getting work for it, as that is a thing that the server should read and decide upon not to send you work. - Edit: having looked that up, that's only done for CPUs, not for GPUs.

But in any case, best set in project preferences not to use the ATI GPU.

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Ok, thanks!