I understand what you're getting at. But the map is not the only problem. Many particle effects do not look as they should. I've read posts from people who had this bug in vanilla Skyrim, without any mods. As I said, I have no idea why Apocalypse would cause something like this. But let me put it in Septimus' words: Now the madness begins.
My strange bug is gone. However, I do not understand the way I solved the problem, because it makes absolutely no sense. So far I have had the bug in all saves, even in those that were created before it appeared for the first time. Even in newly started games. Restarting Skyrim or the PC had no effect. Now I uninstalled Apocalypse, loaded a save, checked if the problem was really gone (it was) and created a new savegame. Then I reinstalled Apocalypse, loaded the new save... and the bug was still gone. But not just in the new savegame. In was gone in ALL savegames, even in those in which it was previously present. What the hell? Since 2011 I have loved and hated you, my precious Skyrim.
After everything I've been able to find out so far, I can't help but think that this is a very elusive bug that no one has seriously analyzed yet.