Does this occur anytime you try and access the forum page?
if so, 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
No it is not theme related. Just decided to install BBPress plugin on my website and get same criticall error. Unpublished every plugin, except BBPress and activated default theme 21. Same problem – critical error.Version 2.6.14
I figured out my mistake. I installed the plugin, created a menu http://www.domain/forums, but did not create a page with the forum’s name and shortcode. Once, everything worked.
Hope will help…