Sorry for bumping up this old thread, but i'm facing the same problem.
When playing on the Sittingen map and the Daytonford map, certain tiles refuse to load.
I've tried reinstalling Omsi and reinstalling both maps, but the problem still occurs.
Does anyone have a solution for this problem?
For some magical reason, the tiles have just re-appeared.
Though it strikes me as kind of odd, im happy the map is working again