Despite running
composer require illuminate/filesystem:8.47.* -W
and downgrading that package I still get the same error.
Edit: ok something like restarting php-fpm seems to have fixed it :-/
Edit: clearing the cache from the terminal still fails with the same error though. The Forum itself seems to be working.
Edit: Hmm... maybe a specific issue here, but the forum seems to only work with php7.4 if I try with php8 it breaks again (I had it downgraded to see if that was the cause previously).