The published version of a game is a little different to your development version, the latter one has a few more things for debugging active and so on. This is why errors can appear (immediately) in either one of those versions while not appearing in the other, although the faulty line of code or damaged file is in both of them.

It was an old/ faulty *.wad file I think which caused the problem. The error appeared first in the published version but after changing and adding stuff (in code) it appeared in the development version, too.