Stumbled upon a strange issue back in February and the issue came to our attention once again these days. It has to do with using a secondary language for your users, so when you only use english in your system, the following info won't help you. Basically if you get the error Stopped (error): User group could not be found one of the problems can be found in a standard class.
Check to see if in the SysWorkflowProviderService method resolveParticipant the _participant parameter received is in english or your secondary language. It may be so that you will get the role User not in english; this will only cause the if on the line 99 to not evaluate to true and this in turn will error out later.