Ubuntu has some mistake. 18.04.
Did a normal update and now dmesg reports:


[515551.148450] NVRM: API mismatch: the client has the version 390.77, but
NVRM: this kernel module has the version 390.48. Please
NVRM: make sure that this kernel module and all NVIDIA driver
NVRM: components have the same version.
[515931.257453] NVRM: API mismatch: the client has the version 390.77, but
NVRM: this kernel module has the version 390.48. Please
NVRM: make sure that this kernel module and all NVIDIA driver
NVRM: components have the same version.

400+ gpu tasks errored off. Well, drat.

For now just no gpu on the machine, I hope uninstalling nvidia,
running nouveau briefly, and reinstall nvidia driver (and opencl part)
will make this annoyance go away. Nobody appears to have reported
this anywhere on askubuntu.

Something is confused. I would
sudo apt-get purge nvidia*
sudo apt-get autoremove

That will get you back to nouveau drivers. Then reinstall nvidia drivers.
sudo ubuntu-drivers autoinstall
sudo reboot

That will get you the standard package of drivers. If that doesn't pick up your Nvidia driver, then try
sudo apt-get install nvidia-driver-390

Hopefully that will get your proper 390 package.

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Seti@Home classic workunits:20,676 CPU time:74,226 hours

A proud member of the OFA (Old Farts Association)

Thanks Keith, I have been working toward that, and it seems to have succeeded.
No more worrying messages in dmesg output.
The sequence:
In additional drivers, select nouveau driver. Apply.
sudo apt-get purge 'nvidia*'
sudo reboot
sudo apt purge 'libnvidia*'
( similar sudo purge '*nvidia*'' did not work to remove libnvidia, it seems)
sudo apt autoremove (that removed a fair amount of old stuff!)
dpkg -l |grep -i nvidia (to be sure all nvidia gone before next step)
In additional drivers, selected 390, applied
sudo apt install ocl-icd-opencl-dev
reboot

Whew. We'll see if that's sufficient. Looks promising so far,
I have new GPU tasks, one running now. I'll watch the error count.

Sounds good. I rarely have issues getting the OpenCL component of the drivers. I think I had to install it manually only once. I do use the ppa however.

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Seti@Home classic workunits:20,676 CPU time:74,226 hours

A proud member of the OFA (Old Farts Association)