Hi,
we have a weekly activity report with jpgs from our club.
To ease the upload - just ftp to a folder - I would like to prepare all folders in advance and give the folder the naming "Mühlenputz" as wanted and the date as the activity is like "2017-05-06".
All empty folders should be invisible for guests.

Unpublish the folders (we call them "albums") so only logged in users with the appropiate rights will see them unless they know the link.
http://www.zenphoto.org/news/an-overview-of-zenphoto-users/

You can also set unpublished as the default state for newly discovered albums on Options > Gallery.

I see from your answer you got not my intension.

I would like to prepare several empty albums - with a different name as the folder.
One album for each Saturday = 52 per year.

Invisible until the first content a jpg is stored to the folder, which should contain all jpgs take during the week.

Probably I did indeed not understand ;-)

I would like to prepare several empty albums - with a different name as the folder.

Maybe you have a misunderstanding here. Zenphoto's gallery is file system based so the folder name is the name of an album (plus parent levels). Not to be confused with the title of an album which is independent of the name.

You cannot create empty albums via the backend upload except using the elFinder plugin which is a way to directly edit (this bypasses certain gallery functions). Or you could do that via FTP or via the object model via code.

There is also no facility to publish albums once they have content. It would probably be possible but you would have to create e.g. a plugin using some filter hooks to do so.

Edit: Seems there is a third party plugin doing this:
http://www.zenphoto.org/news/unpublish_empty_albums/
Never tried it and cannot tell if it still works.