BuddyPress.org

If BP works fine with the default theme and all plugins turned off except BP then it is either a custom theme issue, a conflict with a 3rd-party plugin, or both.

By the way, this also means temporarily disabling any custom code you may have placed in bp-custom.php. You need to distill your setup down to the lowest common denominator and then add back–one at a time–a new variable (such as the next plugin or a custom theme) until the problem returns.