I found what I was looking for here:
https://bbpress.org/forums/topic/bbp_wp_login_action-question
There’s a bug in bbp-common-template.php
Thanks for this. Obviously the = assignment is the culprit, I changed that and it now works for me. Changing || to && does fix it but might break something else, I’m not sure what the logic is but definitely the single = is wrong.
PS is part of the solution in 2.0.2? I just noticed the release notes. I used the templates from 2.0.1