I've been having this problem for quite a while now, I think since Elsweyr dropped.
I'll get home from work, try to log in and be told I'm already logged in. Why ESO isn't able to log your account out automatically upon re-logging I don't know, but anyway.
I literally haven't been logged in for at least 8 hours.
But here's the thing: I have 3 accounts and if I log in on one of my other accounts and then immediately log out and try my main account again it'll log in fine.
So my guess is there's an issue with the client where it's not properly marking an account as logged out, and logging in on another account clears it.
Lucky I have multiple accounts or else I'd be constantly raising tickets.