I have the same problem. But "new topic" is working after reloading the page.
Same thing with the correct theme display of forum topics.
Update. After clean cache all works fine, sorry
Every time I even click on a checkbox to enable a permission for any group I constantly get this error.
11/8 00:46  - [31merror[39m: [groups.join] Could not create new hidden group: Group name too long
Even if I create a group with a shorter name I get this error could someone help me out here?