It appears that, on very rare occasions, there are issues with the login to the BulletinBoard (Forum, BBpress). The login itself is working – however you cant post cause there is still the status:
You must be logged in to create new topics. (or replys)
If you experience this behaviour please reload the page (“shift”-“reload”), use Chrome or Firefox and check if there are custom settings active in the Cache-configuration of your Browser, overriding the server-headers.
IMO the server itself is sending the right (non cache) header.
Best regards, Christian