I've suffered with the blue battlemap problem before. It comes as a result of playing around with vegetation and its generation.
However, this time I have not played around with that.
It only happens when a certain unit of a certain faction is present - a chariot - but only if there are a good number of other units present from that faction too. It does not occur if I fill the whole army with the chariots, but if I pick the chariot and another unit and then the chariot again and so forth, then I get the blue-sky terrain battlemap ;)
I'm a little perplexed as to what could be causing it. I have it confirmed on another PC too.
Just posting this on the offchance that someone else has suffered the same problem and might be able to help.
Bookmarks