Using 3.5.1. The topic was initially in a category that was requiring a minimum of 3 tags, each from a different tag group. I (admin) created the topic in said category, with a single post, and had 3 tags respecting the category’s tag requirements. Then I made it a personal message, which indeed made it a PM to myself. Then I wanted to edit the post body, changed a few words, but when pressing Save Edit I got the error “An error occurred: You’re not allowed to tag topics” … and nothing was saved.
Sounds like a bug
Furthermore, trying to Make Public but into a different category seems like it’s working, there is no error, and the website reloads showing the topic, and the new category shows underneath the title. However, when I click the category to see its list of topics, that particular topic is nowhere to be found. When I go back to my PMs, the PM is still there. So it didn’t actually make the topic public
Sounds like another bug
If I Make Public into the old category, then it does move it back (topic is in the category’s list of topics, and goen from PMs).
If I delete all tags, then Make PM, then Edit, then it finally works.