No Access to Network Setup

Hi everybody, I can’t find a solution for the following problem:

(All error-messages and menu-items are re-translated from the german wp i use, so the exact words probably are not the same than in the english WP)

I’ve a multisite-istallatione with several domains. At first, everything worked as expected. After doing some changes at the domain-mapping, some updates etc. I still could work without any problem on the singe sites / domains. Now I realized, that I can’t access the network setup anymore. Under “my websites” I see all sites, but the menu-item “network setup” is missing. So I can’t install any plugins, change the networksetup and so on.

If i call https://www.mydomain.de/wp-admin/network, i get this error:
“You’re not allowed to access this website”

Bacause it’s a few weeks (or month?) ago I made the mentionned updates / changes and didn’ use the network-setup for a while, I don’t know, what exactly caused the problem.

So the question is: How can I get access to the network setup?

I hope someone has the right hint …

Best regards

Uli