Sometimes when I alt-tab during a mission (I play in fullscreen) to google something or look something up on the wiki the game disconnects me. I'm just looking at the wiki on Chrome and the game icon on the taskbar disappears. When I try to log back on the start menu freezes and I can't log back on until it kicks me out of the mission. I've died before on missions and I've gotten disconnected when the other person got on the elevator and it still wouldn't let me log back on. I've lost boss rewards because of this problem. Anybody else have it?
How are you playing? through the web client or through Steam? When playing on the web client, you will be disconected if you are away from the game for a period of time. I think 5 minutes or if you don't have steam overlay checked (playing on steam). At least that what i think your issue is.