Hm... I think I - after jcl told me that the reason very likely could be a faulty texture - just took the level, deleted maybe half of it and tried again. No error -> deleted less and tried again, until I had it isolated (it was the r25.wad if I remember correctly, should anyone else be using that).

When you have let's say such a faulty texture loading that may not cause a crash, but some other object that gets created later could write into invalid memory areas because of that - at least to my knowledge.