[_ck_]

Was your bbpress setup before you installed WordPress?

Or did WordPress exist before bbPress? If you had bbPress first, the userbase is actually inside bbPress and not WordPress. It needs to be moved to WordPress (or some different settings in WordPress to use the user db in bbPress). I call that “reverse integration”.

But I’m missing a detail here – did you have working integration with WordPress+bbPress before you upgraded both to latest?

This might be an issue with the two parts of the “secret key” not being setup/synced properly between the two.

I think this is a similar problem posted here (that was never answered)

https://bbpress.org/forums/topic/bbpress-09-released#post-14973