What sovereign are you choosing, try removing your old units, races and sovereigns, and play with a premade, if that works you can probably create a new custom sovereign again.
Sometimes outdated sovereigns can cause problems too.
If this is not the case, sorry.
Also, can take a look at this: https://forums.elementalgame.com/415301
It explains how you gather the Debug.Err and another file that helps debugging the game 
Sincerely
~ Kongdej
