ok, so if you can’t the fields, that means they aren’t being saved.
it could be a theme or plugin issue
Themes
As a test switch to a default theme such as twentytwenty, and see if this fixes.
Plugins
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