I get the same error from your link. However, I've discovered I can see the subforum when using as a guest in incognito...
ETA1: And if I log in in incognito, the subforum disappears. It really does behave as though I've been banned from that subforum.
ETA2: The problem persists while tethering from my phone, which suggests it's not linked to my IP address. (I know there's some known issue with users getting incorrect ban notices according to IP address, so thought I'd check.)