. When you leave your browser up on your desk or even just a tab open and it exits out from the forum automatically or just the browser stopped working because of inactivity and then you try to go back on and see that message and have to log back in. I know it can't just be me lmao
I haven't really experienced such before, the only message I get sometime is when I log into a new browser and forget to hit the stay logged in button. So I get to view the forum as a guest until I login again.

My Bitcointalk account keeps logout even if I click that remember me box when I close the Bitcointalk tab and turn off WiFi. But user doesn’t exist error is due to wrong profile UID on the link since you can still view the page that you left before your account is logout since forum allowed anonymous browsing.
@OP, I think the error message that you get is when you are on the middle of creating a thread while your account get logout due to inactivity. This happened to me many times because I’m very slow to create a thread.