Please check if the file map_istaria_resource.def in the folder resources_override\interface\maps\istaria exists.
If it exists, please check its size, it should be 138kb.

If it doesnt exist or it is the wrong size then please redownload map pack and reinstall.
Ok, it exists, right where its suppose to be and is the right size.

Isn't that intersting. Any other ideas or should I just delete and start over?