Okay, this just happened to me AGAIN. I was just in Baron's Retreat in Mirage. When I got to the last boss, I marked my spot and ported to get health and mana. I wasn't even gone for maybe 30 seconds, but when I tried to teleport back, I got the message saying that the dungeon timed out and I lost my progress. This is SO frustrating! I advance in the quest up to the boss fight, but still have to go through the entire dungeon. This is the second time this has happened in Mirage, please fix this. Soon.
Some dungeons won't let you return. I got killed at the Lt. Culpepper tower, and tried to port back in to my teammates, to finish. It wouldn't let me into the room again :(