Thanks for investigating where the issue was coming from.
When set to true, the multiPartModel property indicates that a 3D model file should be saved with all the files stored in the same directory as the model (like MTL or texture images). But if this property is wrongly set to true and the 3D model file isn't in a directory, it will give the error you reported. Therefore, I updated the Beta version 4.2 to silently accept that kind of 3D model at saving time, ignoring the fact that it should be in a directory of the sh3f file.