Oracle has the following recommendations if you run into this especially after a PeopleTools upgrade. The first step would be to make sure your PeopleTools patch was applied correctly and possibly reapply it if you think it wasn’t installed properly. Redeploying PIA is something else I would recommend if your fighting this problem. Clear all your cache, server and client/browser side.
I saw Oracle now has a case online indicating a problem with PeopleTools patch installations becoming corrupted and their first recommendation is to reapply the patch.