When playing the epic prologe a second time the game didn't started the cinematic after I destroyed the blackstone fortress. Instead the explosion was frozen on it's highpoint, but I could still move around the map and select ships, but all UI was disabled and you could see the Phalanx far below the map. I would post a picture, but sadly the Windows 10 screenshot tool only recorded a black screen.
It seems that i have destroyed an other chaos ship a splitsecond after the fortress, maybe this could be the reason for this bug.