Thank you for your reply.
Maybe I did not make clear enough, that I can already see the forums by doing what I described before.
The problem is with the e.g. profile view.
The plugin you are suggesting does indeed restore the profile view with the “Theme support” enabled. – But the forum view has an additional headline added and my footer is getting messed up. So unfortunately not ideal. – I will try to extract the function for the “Theme support” from the plugin and see, if I can create a more sleek approach, since I do not need all the other stuff that comes with the plugin. – Thanks again for pointing out the plugin, it will help me looking further into this issue!