don't what what's wrong at the moment with Windows. Have to reinstall it again, because after deactivating IPv6 everything is freezing after 2 days of running. Also connection via Remote Desktop is not working anymore. After a restart and logging in into the VM (via Proxmox) it's possible to connect again. Maybe there's a problem with the IP itself, or I did something wrong with deactivating IPv6.
I'll reinstall the server competly with a new IP and a new MAC. Then I will install ALL Windows updates and THEN I will deactivate IPv6. The shittiest thing is I have to pay 0,60 EUR for calling Microsoft to let them activate the license.