WSL ubuntu installation fails; Neworkname is unavailable

After upgrading Windows to 19042 and WSL1 to WSL2, I stuck with the installation of the Ubuntu20.04 distribution.
From my perspective, it should be a completely fresh installation.

But it always shows

Installing, this may take a few minutes...
WslRegisterDistribution failed with error: 0x80070040
Error: 0x80070040 Der angegebene Netzwerkname ist nicht mehr verfügbar.

Translation: The specified network name is no longer available.

I tried to download and install the distribution manually, with the same result.
I tried to import an existing instance of WSL2 Ubuntu18.04 with the same result.
I deleted every directory under %AppData%\local\Packages indicating a Linux distribution.
There is no registered distribution in wsl /l which I could remove via wsl /u. So the installation doesn't seem to reach this point. I removed every previous installed distribution via that way.
I tried to reset the respective distribution I also tried to disable and reenable the WSL-feature in Windows and reboot, without any luck.

Is there someone with a new idea?
Can I somehow reset the complete WSL feature? Maybe there is an option still set from previous tries.