bbPress is tested with wordpress default themes. It maybe a conflict – you need to check plugins and themes
It could be a theme or plugin issue
Plugins
Deactivate all but bbpress and see if this fixes. if it does, re-enable one at a time to see which is causing the error.
Themes
If plugins don’t pinpoint the problem, as a test switch to a default theme such as twentyfifteen, and see if this fixes.
Then come back