bbPress.org

ok, just tested this on my test site, and it works fine.

It could be a theme or plugin issue

Themes

As a test switch to a default theme such as twentyfifteen, 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.

Then come back