I've had this problem too, and it seems to be related (in my case) to a problem with the firewall of this version that occasionally blocks the internet but also avast UI.
Sometimes the message is "UI failed to load" and sometimes "You are not logged in".
Even though avast has given a fix, I'm not sure that this firewall redesign is - so far - fully worked out.
edit:I'll try next time it happens to capture the "you're not logged in" message. I haven't seen it since I've been using avast.