Logins are tracked with session cookies; IP addresses aren't checked at all. (It's normal for multiple people to get the same IP address, or one person's IP addresses to change during a session, whether they use Tor or not, so that would never work in the first place.)
I was also worried that for some reason when I log off I get this error: "Session verification failed. Please try logging out and back in again, and then try again. " It sounds like "your account is still opened, log in again and close it".
It actually means the opposite: that your session cookie is expired or invalid (possibly carried over from a previous session), so you're not actually logged in despite what is displayed in your browser. You need a valid cookie to log out in order to prevent other people from logging you out without your permission (which would be most annoying); the case where you can't log out because you're already logged out doesn't seem to be handled well.