When I copy the an “As shot” setting, it behaves correctly. When I modify the white balance (which then switches to “Manual or custom”), and then copy it, it also works correctly!
However, if I copy the white balance (“Manual or custom”) from an existing image, that has been edited in a previous version Photolab, then it does not work! It will copy completely random values to the new image. So I guess there is something wrong in either the database or the DOP files, that the newest version of PL cannot interact with previous versions anymore.