I think, this wasn’t clear enough: I solved the problem, by setting the eternal storage at the right menu:
Not the user menu: ../index.php/settings/user/externalstorages
But the settings menu: ../index.php/settings/admin/externalstorages
I think form a design perspective this is not ideal, as for a user point of view its not clear – at least not at the first glance – what the difference is.,
However it was my fault…