it could be a theme or plugin issue
As a test switch to a default theme such as twentytwenty, and see if this fixes.
If that doesn’t work, also deactivate all plugins apart from bbpress and see if this fixes. if it does, re-enable one at a time to see which is causing the error.
If you cannot do this to your site (say because it is live) then use the ‘troubleshooting’ features of this plugin to let you test without affecting other users
Health Check & Troubleshooting
Then come back