After having installed phpBB SEO premod 3.0.12, I've configured in the ACP (as told in the instructions) my phpbb SEO class settings. After that, I renewed my .htaccess and put it via FTP in the root file of my forum installation: mydomain.tld/forum/.
I still get 404 errors when clicking on a new registered user or a newly created forum. Someone an idea why? (mod_rewrite and so on are on...)