I've been playing the first Thieves Guild quest this morning, for 2 hours. I got up to the point where I had stolen the skull and was in the catacombs. I had to log out to go and do some real-world stuff, and when I've logged in again, I'm back at the Woodhearth Docks next to the boat that takes me to the quest. If I get in the boat, it takes me back to the very beginning of the quest. When I load ESO it should take me straight back to the location I was at. I also have everything I stole on the quest up to that point, so it looks like it has reset my location and quest progress, and dumped me back at Woodhearth with my full stolen inventory.
I've almost started this quest a few times, logging out when I'm on the dock at Fulstrom Manor, and each time I log in I notice I'm back at the dock in Woodherath.