When I was in the Nether and used the portal to get out of it again, my game got stuck on the "Loading Terrain" screen. It wasn't frozen, it still kept running, but after about a minute of loading I got disconnected. And when I went to log back into the realm it got stuck on the "Loading Terrain" screen again, without ever putting me into the world itself. 

I haven't been able to join the realm again since.