I checked my logs to confirm fix, and before updating client build version, the new Core21 work units were all failing. and after updating client the new FahCore21 automatically updates even if your running multiple GPUs,.. if your running single GPU system with older client it would have automatically updated but not if running multiple GPUs, due to the Core can not be updated until ALL the GPUs are not using FahCore21.exe. My old log shows (Core_21.Fah) work units all failed, and now it's showing as (FahCore_21.exe) and with new file the work units are all working fine,.. so seems it was the program data in the FAHClient\cores\fahwebx.stanford.edu file that was borked.
So lesson learned, always run most current client version, and current GPU drivers for best Folding performance. I would always update the GPU drivers when new updates were released, but never thought about updating the FAH Client Version.
Edited by Braegnok, 13 January 2019 - 08:05 PM.