Dark screen.

Here's a similar issue, by the way – when attacking a Castle, sometimes the game just gets stuck on a black screen. You can hear the unit sounds and the music, but the screen remains black. I'm playing with the Conflux expansion, version 2.1.1, and I have all the expansions purchased. This is the Ubisoft version (not Steam). It's good that it doesn't happen often, but it still occurs occasionally.