Answer
Confirm the user has the FND_ENDUSER permission set granted — within its own grant structure, FND_ENDUSER includes another permission set, FND_RUNTIME, which is what actually grants the ClientApplication activity. If the error persists even with FND_ENDUSER granted, check that FND_RUNTIME is still correctly included in FND_ENDUSER's grant structure and that ClientApplication is still granted within FND_RUNTIME — these are core Foundation1 permission sets that should never be manually altered, so an unexpected gap there usually points to an accidental modification rather than something that needs a new grant added.