The quickest way will be to turn on debugging in wordpress and come back with tne exact error you are getting
https://docs.themeisle.com/article/819-how-to-turn-on-off-debugging-in-wordpress
If you can’t do that, then 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
Health Check & Troubleshooting
Then come back