I think I found the problem. It seems like we were playing from different versions of the game. The save was sent by a (very thorough) beta tester, and I tried to load it from the final version. I don't have the beta version easily accessible at the moment, but I guess that (slight) differences between two versions can make the saves incompatible? However, something tells me that this shouldn't happen since I constantly use the same saves in my working directory during production throughout numerous changes, without problems. But maybe this is not possible with compiled games?