I surfed a lot about this and I am almost sure that it is not possible because of how cookies work, but I want to ask anyway:
Can I run WP phpBB Bridge in a multi domain setup?
If yes, how can I achieve this?
If not, I think an idea to run WP phpBB Bridge in a different setup, like keeping the login page of WordPress active and just syncing the users from phpBB to WordPress, also creating the users on phpBB, would be a real benefit. I noticed many sites where the webpage itself and the forum are seperated on two different domains, not just the same domain or several subdomains of the same domain.