I have this too. I think this happens if a battle starts in morning or evening. I have had 2 Battles like this.
I have found the problem with the savegames:
I have modded the EDU (Cavallry). In other Mods like Third Age or S&S i can make changes without crashing my old saves. With this Mod it is not possible to do that. I do not know why.
So with the modded files i can not start one save because i have changed the mod after starting the campaign with wessex. With the Vanilia-Files i can load the Normandy-Campaign.
Now i am playing Denmark and unitl now all is ok. I am Saving and reloading every round and all is ok.