I just found out, it’s the forumpress plugin
deactivate the plugin, at least that will bring your site back to work
for more see their support site (seems they haven’t yet found a solution):
https://forumpress.org/support/forumpress-g5/forum-server-1.5-f29/broken-forum-when-upgrading-to-version-1.6.8–t659#postid-2384
