[_ck_]

Note there’s a similar (but slightly different situation) here:

https://bbpress.org/forums/topic/key-master-is-not-allowed-to-add-forums?replies=3

and here:

https://bbpress.org/forums/topic/key-master-doesnt-have-permission-to-add-forum

Are you using the “use display name” plugin?

That may be the culprit, try deactivating.