Problem with Block themes
I tested wpforo with twentytwentytwo theme
it is based on new theme structure (Block theme)
It seems that it is not compatible with this new theme type
I get wpforo 404 error when I visit forum links
Works very well with those kind of themes.
Here is MY staging server with the theme you mentioned and wpForo plugin.
Your 404 issue is something else, related to your setup or settings.