No answer to this problem yet? I suddenly found the same problem. Besides I have the “ERROR: This forum is private and you do not have the capability to read or create new replies in it.” but the strange thing is that I’m logged in as Keymaster :-/
Additionally I got issues with URLs because URLs for forums give a 404 error but URLs for topics do not :-/
yes actually the problem finally went away I’m trying to remember what it was that fixed it…
it was either an upgrade or this thing i think:
https://wordpress.org/plugins/bbp-capabilities/