It could be that there's a bad lightmap... or it could be something totally unrelated.
A possible way to get this fixed is to go to the area you were, push the "x" button to get the area code(say in this example "arvwdf"), and then exit the game, search the game folder for the file(the example it would be arvwdflm.bmp) and it's backups from the mod folders and put them one by one to the game folders override folder.
This should work so long as you have not deleted the mod folders.