Hi Gӧtz,
did you have any problems saving that project? I mean that the first attempt failed e.g. due to insufficient disk space or something similar?
Similar problem has been reported to us once. In the project file there were incorrect paths to some parts of the project probably caused by the incorrect saving during the first attempt. However, we were not able to reproduce the problem.
Can you please try to change the paths in the project file and open again (without the old project folder)?