After upgrading from 14.3 to 15.3, we are getting lots of below error messages in bg-ca.log:
With 15.3 there is a new feature where the BPM_ERRORS table is backed up and cleared out. So, I don't suspect that you would have any completed or old process instances lying in the system. But just in case, check those and delete them.
Also, restart the BG service and check if the Errors still come up.
If you still get the errors, you can raise a case with CA support and we could try to find the cause.