Sorry to necro this, but I found this thread via Google yesterday and had the same problem. I've since solved it and thought I'd share it for anyone else who finds this.
So this issue cropped after I brought across map files I have been making with Bare Geomod, into another mod, a copy of the Americas expansion campaign.
The pattern proved to be different depending on whether my map_fog.tga in the base folder was all white (255 255 255):
Or all black (0 0 0):
It was the same for all factions and covered the entire map with a pattern, with the exception of the bottom one-third of the map. Error logs showed nothing.
The bottom one-third was the clue. My tall map was "pasted" over Americas' short map, so a file somewhere must still be in the shape of the Americas' map.
After a bit of headache, I checked the map files in both base and imperial_campaign again, and sure enough, this time I noticed another map_fog.tga, but in the imperial_campaign folder, not base. I'm not sure what it was doing there, but it was a leftover from the Americas' files, and sure enough, once I deleted it, the issue went away.
I'm unsure if this solution would have been applicable to the OP, and being one year ago it's probably moot by now, but I thought I'd share what I found anyway.