I've experienced contradictory form behaviour the past few days. It is usually solved by logging out then logging back in.
There are times when the forum shows I'm logged in (the top banner), and other parts of the forum act as I am logged out - unable to post reply, no indication of read/unread posts.
Today just typed a long reply, but the forum would not allow posting using the "Post" button. Flipping back and forth between Chrome and Firefox seem to fix that.
I've cleared the browser cache on all installed browsers. The symptoms have persisted the past few days. Using Windows 10 (1909), Firefox 73, Chrome 80.0.3987.106