After waiting a long time I got an answer back from vmware engineering:
" user needs ReadOnly permission on the VC level. This is required for other PowerCLI functionalities like Tagging,ContentLibrary etc."
In my opinion, this really makes no sense, because as I told them: it works with an older PCLI Version, with REST and the with the GUI.
And we would and could not give everyone Permission on VC Level.
I will keep this updated.