Many Thanks Juri,

The quickfix works although it could not verify 2 signatures:

gpg: key E1F958385BFE2B6E: 2 signatures not checked due to missing keys
gpg: key E1F958385BFE2B6E: public key "X2go Debian/Ubuntu Packaging debian@x2go.org" imported
gpg: Total number processed: 1
gpg:               imported: 1

Do you think this can lead to a security issue due to the 2 signatures not checked?

Best Regards, Saion

On Wed, 28 Jul 2021 21:40:49 +0200 Juri Grabowski <x2go-dev@jugra.de> wrote:
> Hello Saion,
>
> as a quickfix you can use another gpg server:
> apt-key adv --recv-keys --keyserver keyserver.ubuntu.com E1F958385BFE2B6E
>
> On debian buster-backports and newer systems you can use extrepo:
> apt install extrepo && extrepo enable x2go && apt-get update && apt-get install x2goserver
>
> I have changed this in wiki.
>
> Best Regards,
> Juri Grabowski
>