Red Spot, when you get the CTD you are probably finding a variation on the landmass problem;

See discussion on landmass problem here
and me attempting to summarise it here,

and yep that is a complete pain in the .... to work out, I had one map that worked if it was used one way but wouldn't if it was simply flipped over so left hand side went to right!

I really must try and write that up properly - but basically the impassable bits of land mountains and forests etc. can cause the same CTD problem as too large an area of sea. If your impassable land adjoins a sea area, the impassable land area and sea join together to form one large impassable area giving same problem as if both were sea! (did that make any sense?)

Your freeze with the addition of more fertile areas sounds like you might have found something new as well though... when you're experimenting use a beach ground_type or something that is passable but not fertile, to test if it is actually the fertility or the impassibility that is issue.