All these problem are both cookie path
and forbidden character problems.
Both are poorly documented (or not mentioned at all)
but the good news is that fixes exist for both issues.
I have to find my previous instructions on this but
because bbpress search is so poor, this may take awhile.
Could someone please get this into a wiki or faq at some point:
Steps required for more complete and problem-free bbPress+WordPress integration:
1. cookies paths need to be changed to the same path in both WordPress and bbPress (change to domain root recommended)
https://bbpress.org/forums/topic/wordpressbbpress-single-sign-on?replies=22#post-9010
2. the integration plugin should be installed
https://bbpress.org/plugins/topic/4?replies=26&more=1
3. the allow spaces in usernames plugin should be installed (also add my underscore hack for characters bbPress by default forbids but WordPress allows – see my message later in same thread)
https://bbpress.org/forums/topic/usernames-with-spaces-do-not-work?replies=28#post-7904
4. add the ” set default role when registering new user though forum” mini plugin should be installed:
https://bbpress.org/forums/topic/yet-another-integration-bug-no-role-set-on-registration?replies=10#post-8768
5. search your bbpress templates and force all registrations/logins through wordpress from the several places they are buried on the forum (the WP login interfaces are far more developed) – unfortunately at least one bbPress login location is hard coded in the core which will require a direct hack everytime you upgrade. ie:
<?php $request_uri=$GLOBALS["HTTP_SERVER_VARS"]["REQUEST_URI"]; ?>
<a href="/wordpress/wp-login.php?redirect_to=<?php echo $request_uri; ?>"><?php _e('Login'); ?></a>
<a href="/wordpress/wp-register.php?redirect_to=<?php echo $request_uri; ?>"><?php _e('Register'); ?></a>
6. optional but highly recommended, install the “use display name” plugin for mods+admin:
https://bbpress.org/plugins/topic/2?replies=6&more=1