Yeah seems like anything guild-related in the chat is borked.
I got the “You are not in a guild” error despite being in one. I did the offline->online solution, and got my chat to work.
However, something is amiss. Whenever I chat with the guild (in the general chat tab), the Guild tab in the chat always glows (normally it only does that when new chat entries happen while the chat is scrolled up). So then I switch to that tab since it’s irritating to see it glow up. Then I went back to the general chat tab, suddenly the posts I used under the /guild chat vanished.
Holy kitten!