I noticed that there is always only one "Drawn Contour" object stored in the first Label. So after Reload the interpolation works only for the first label.
Anyway, I wonder what is the right workflow then. I initially thought that the interpolation is only used at the very end where I press confirm and the final segmentation is generated.
Some organs like the skull are rather complex and a correct interpolation is maybe just generated after a few days.
If reloading interpolations does not work on bigger projects, one is forced to use confirm interpolations on small sub-segments before saving the project.
This is a bad idea, because one can hardly modify/correct them later. Consequently, the best workaround seems to save projects with contours enabled with just one label :/