Blue battlemap problem - no, not the usual

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.

wierd

All my problems are weird. It's the Orcs y'know...they get in the hardware and that's it... :(