Fixed the issue I was having with this same error.
Whew… after lots of searching and turning plugins on and off, I found the conflict and it was actually just settings that were defaulted incorrectly the Ultimate Member plugin with it’s add-on Extension for BBPress (premium plugin from Ultimate Member extensions bundle).
The Ultimate Member BBPress extension has settings for every user role in the Ultimate Member > User Roles section, and in those settings it has permissions for BBPress that override BBPress settings. In those settings there are checkboxes for each permission, including posting replies, posting topics, etc. Customer roles, admin roles, and a bunch of other roles were for some reason defaulted to not allow these types of posts.
To fix Ultimate Member permission settings, go into each role and enable the features / permissions accordingly.
