Dear both,I changed the theme and now everything works!Thanks a lot for your help!Kind regards,Kevin

The forum does work again (I had to reset some permalinks).However, I still can't edit/delete topics and posts..

Dear Sofy,Thank you for the support.I did the following:- installed SSL,- disabled most of the plugins (ones mentioned by dimalifragis)- reinstalled t...

Thank you for the reply.I indeed did exclude WPForo from the Caching plugin. how ever I now disabled and deleted the plugin together with 3 other plug...