Hello, Tamriel. I'm a longtime reader of the forums and a frequent visitor of Cyrodiil and Wrothgar.
So like many of us, I started experiencing some pretty difficult connection issues. In my case it began in the spring. That handy optional latency indicator in the UI would hit the mid hundreds or 999+ and turn red, movement of all other actors would cease, and inevitably one of two things would happen. There would be a flurry of movement on my screen as my end caught up with the server, or I would simply be kicked to login without an error message, while my character remained logged in.
As time went on, I started getting sent back to login without fail. It didn't seem to matter if I was in a crowded starter island, a fairly empty Cadwell's Gold map, Wrothgar, or Cyrodiil. The disconnections, which happened anywhere from every five to twenty minutes on a bad day, were inescapable.
My partner, who signed up for ESO along with me, has never been plagued by this matter. In fact, she guards my character until I have been fully logged out, or I manage to return, whichever comes first.
We considered my Internet, but what baffled us was that I did not seem to have these issues at other times. But I soon started having difficulties with other multiplayer games and Steam in general, and elsewhere in the house, my sister kept getting disconnected from her own MMO sessions.
The next things we thought of were the modem and router, but both were just fine. My router maintained a strong connection to the modem, and during all these issues, the modem never lost its connection to the Internet. And that one observation helped me narrow things down.
I installed a little program to monitor for connection failures. And as it turns out, there were often several each hour, each lasting for ten to twenty seconds. Especially during peak hours. The failure alerts would coincide with disconnections from not only ESO, but various programs that needed a constant connection. Not just for me, but for the rest of my family.
In the end, my ISP was contacted and the constant drops were reported. And what I learned was that it wasn't just my home having this issue; the entire area was having this problem. They were working on a fix.
I'm not sure what they did, but it's been a few days and so far my disconnections from anything, ESO included, have been rare. They haven't stopped completely, but I can usually make it through an evening of gaming without getting kicked.
tl;dr: It seems a hiccup in an Internet connection can cause ESO to return to login. It's definitely not a fun solution or a quick fix, but if you've exhausted all your other options through troubleshooting, then checking in with your ISP might help.