"You are not in a guild."
I just actually made mine work, by switching myself to invisible in the friends tab, and then back to online mode. Doesn’t fix the overall issue, but put me back in contact with my guild at least!
We’re seeing this, too. Between this and the horrible lag (while doing events in Groznev Delve, which is a bad place to be lagging out) it’s time to call it a night.
I have this problem, too. Toggling invisible and back doesn’t fix the problem, for me. My other guild chats are working, but my main guild says, “You are not in a guild.”