I've noticed while reviewing some other logs that the message "StatusFormatter::getMessage: Invalid result object: no error text but not OK" is logged a lot on the WikiEditor channel by the new code added in T410877. https://logstash.wikimedia.org/goto/04da6c22a1d0e9c7d75af8cc363297df

So, I reviewed everything that could conceivably produce such a status (Codesearch), noticed and confirmed locally that this happens when you get an edit conflict. A few other places in EditPage also use such statuses (see T384399 and that Codesearch link), but they should be a lot less common.