Digged a little further into this So in his Cabinet is a shared folder called Suplliers. He is not the owner of this folder. In this folder are a lot of sub folders, 1 for every supplier. There are 105 subfolders inside the Suppliers folder.
Now his collegue sends him a new subfolder. He tries to accept the new subfolder and tries to navigate it into the supplier folder. This does not work, he cannot click "to the right" if you know what I mean.
So I have now put them just in the Cabinet, and what do you know ? From there I can move them into the Suppliers folder !? Ok so there seems to be a minor bug in the folder placement part. Problem solved, thread closed, user happy :D