Arkshine found out it was because of Multilingual Plugin that is (of course) still using the former CSS way of splitting tags with commas, while new Discourse version now uses a HTML way of splitting tags with commas.
@Zas, is this the plugin you mentioned you still had to upgrade, in MB meeting?
zas disabled the multilingual plugin again, hence the issue is gone.
That plugin really seems to be frequent cause of trouble. I actually don’t think it is really worth it, as the forums are primarily English. Even the occasional posts in other languages work usually well even without explicit support by the forums.