I'm not sure what happened before, but this symptom is from the topic title change.
Exception Caught: Conversion from 'topic-title-wikitext' to 'html' was requested, but this is not supported. Edit a previous post: An error occurred.<br />The error message received was: internal_api_error_Flow\Exception\WikitextException
While verifying T116095: Ensure users can not access topics from deleted boards via Varnish caching found this edge case:
- Log in as a non-privileged user and go to any Flow board page.
- Log in as Admin and go to the same Flow page that a non-privileged user is currently viewing.
- Admin deletes a topic - a non-privileged user attempts to edit or to post a reply to a deleted topic. A user receives quite few error messages (some of them malformed) and none of them is helpful.