This happened at our company. We got the error because we had changed some of our users' names. One of the users to which the the approval rule pointed was no longer valid, so it locked access to the Manage Purchasing Document Approvals screen. We temporarily change the user's name back to its original name. We were then able to get back into the Manage Purchasing Document Approvals screen... then it was a shell game. We gave the rule a random user, saved, changed the approval user back to its new name, then assigned the the approval user to the approval rule.