In case you're facing a C4C dump (500 Internal Server Error), create an incident from it and find that the the Incident Context => /SAP/ABAP/RuntimeError file shows the error "Object instance creation of action rejection implementation class "<PDI solution namespace>_CL_ACTION_VALIDATION" of action "<PDI solution namespace>_<ID>" on node "ROOT" in BO "<BO>" failed" - you have to do a full solution activation to resolve the issue.
According to SAP Support "sometimes the class generated can be only synced by activating solution.". I've faced this error twice recently but never before, so it appears to be a new bug in C4C.