This could be an issue with your rewrite rules. To fix this try resetting your permalinks. In your WordPress Administration Screens navigate to Settings > Permalinks, select a different permalink structure and save. Then select your preferred permalink structure and save again.
It could be a theme or plugin issue
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.
If plugins don’t pinpoint the problem, as a test switch to a default theme such as twentyfifteen, and see if this fixes.