This issue could or could not be related to the software. First run File health check in the admin control panel if you are able to and see if the files are all there. Second post any log errors if any. Third try to re-upload XF and see if its going to solve the issue. It looks like permission issue to me, check if you have all privileges of a super admin. Fifth check console errors if any. As an admin check if you can post a thread or make a reply, upload an avatar, make use of XF forum functionality. If you can share an admin account via PM I might be able to see if its an XF related issue or not.