bbPress.org

Hi,

Most probably a security issue or a redirect to your login page that does not work. If you have any security plugins (like Theme my login) or captcha (like in Jetpack), try to deactivate. Somebody found this: https://bbpress.org/forums/topic/users-unable-to-register-and-login/

Just be careful : Editing .htaccess can block your complete site. Make sure you have a contact point with your provider and you know what you are doing

Defaults for the .htaccess can be found here: https://codex.wordpress.org/htaccess

Let me know if it helped,
Pascal.