Posting the condition definition and the section in the process where you are using this condition might help in the diagnosis. The error in window manager doesnt give a lot to go on IMHO.
I've seen a problem similar in the past where there were 2 preconditions on the same status (before manual actions) with mutually exclusive conditions which created something similar at runtime. Probably not the case here but seeing the process section (all parts leading to from the status that is before this) may assist in diagnosis.
After all the hassle, finally found a workaround to it. it turns out that the system can't have the manual action Add Assignment under the pre-condition that check for 'Is current assignment group'.(Object attributes=current assignment, Attributes =Group, condition = Equals, runtime value = $currentgroup$). There seems to be a conflict getting the business function to work.
The other processes that passes the pre-condition 'Is current assignment group' in other processes is followed on with a different action such as "Acknowledge", "Approved" and not 'Add Assignment'.
I having the same issue. Could you please explain which was your work around?