It has come to my attention that there appears to be a flaw in many, if not all, versions of Nvidia's 295.XX drivers.
If your monitor is put to sleep by the computer, the GPU is no longer available as a CUDA device.
This will cause an immediate failure of any CUDA WUs. This includes GFN/Genefer, PPS Sieve, GCW Sieve, and llrCUDA.
Either roll back to an earlier driver version, or configure your computer to never power down the monitor(s).
My lucky number is 75898524288+1