Something did happend to theses v3.21 task, maybe I did some gpu intensive drawing when these were "stuck"
"CL_OUT_OF_HOST_MEMORY" error on everyone until "No ATI/AMD compatible OpenCL device found." after almost 10 hours of running.
Then several hundred "No ATI/AMD compatible OpenCL device found." error tasks until I rebooted host.
After that, all v3.21 tasks were completed succesfully (except that one invalid task which You did mention)
Then came v4.01 and everyone error while computing.
Sounds odd that hardware error came just when V4.01 came out, this host haven't got single invalid GPU task never before. Is V4.01 harder to GPU that it is malfunctioning when runnning those V4.01 tasks?
And yes, I do know that CPU tasks run faster when GPU is not running.