So I wanted to play a custom battle with the Holy Roman Empire, and got a crash near the loading screen. After using the exclusion method, I came to the conclusion that this crash happened whenever I chose Dismounted Men At Arms with upgraded armour in my army.
I think that the problem also occurs with other units after upgrading their armour, so maybe some of them are missing upgrade models? I've yet to test this out thoroughly though. Haven't tested the HRE in campaign yet, but I thought this could be a problem whenever I want to field Dismounted MAA in a settlement with upgraded smiths. 
My log:
I used SS6.4, Early Era Campaign to receive this crash. Any thoughts or solutions to this would be most welcome.