follow this , it could most likely be a plugin issue
bbPress works with many themes and plugins, but it is impossible to test all combinations, and many issues are caused by conflicts with plugins and themes.
Therefore before posting, please test if your issue is resolved by the following
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 twentytwelve, and see if this fixes.
If the above produces an issue:
If it is a paid theme or plugin, please contact the authors for resolution.
If free, do ask here, we may know of workaround.
PLEASE tell us you have done the tests above when posting.
OTHER COMMON FIXES
The following will do no harm, so are worth a try and can fix some issues
Dashboard>settings>permalinks. Don’t change anything, but just click save. This resets the permalinks, and can solve some 404 errors.
Dashboard>tools>forums>repair forums and run one at a time. Useful if you’ve got some stuff not showing – doesn’t often fix but occasionally does.