Thanks Mohammed for your answer but I know this article and I did it exactly like described.
The problem is that Desktop then shows two datastores in the list.
It seems that all files are correct moved when looking on the datastores, but the hell two entries in the list of the Desktops.
When I try to move back the pool to its original datastore, there is only one datastore in the list.
This is why I think something isn't correct moved. Probably it's only not refreshed correctly in the SQL database.
Please see the screenshot