Receive hooks broken for ssh push

I have postponed the analysis for this problem way too long ...

Steps to reproduce

Note: I am not using the provided PostgreSQL server

mv /opt/gitlab /opt/gitlab.bak
mv /var/opt/gitlab /var/opt/gitlab.bak
mv /var/log/gitlab /var/log/gitlab.bak
mv /etc/gitlab/gitlab.rb /etc/gitlab/gitlab.rb.bak
mv /etc/gitlab/gitlab-secrets.json /etc/gitlab/gitlab-secrets.json.bak

zypper in -f gitlab-ce

   Forcing installation of 'gitlab-ce-12.2.5-ce.0.sles15.x86_64' from repository 'GitLab 15.0'.
   Resolving package dependencies...

   The following package is going to be reinstalled:
     gitlab-ce
   (1/1) Installing: gitlab-ce-12.2.5-ce.0.sles15.x86_64 ....................[done]
   Additional rpm output:
   warning: /var/cache/zypp/packages/x86_64/gitlab-ce-12.2.5-ce.0.sles15.x86_64.rpm: Header V4 RSA/SHA256 Signature, key ID 
   f27eab47: NOKEY
   gitlab preinstall: 
   gitlab preinstall: This node does not appear to be running a database
   gitlab preinstall: Skipping version check, if you think this is an error exit now
   gitlab preinstall: 


   Executing %posttrans scripts .............................................[done]

mv /etc/gitlab/gitlab.rb.bak /etc/gitlab/gitlab.rb
mv /etc/gitlab/gitlab-secrets.json.bak /etc/gitlab/gitlab-secrets.json
gitlab-ctl reconfigure

git --version
git version 2.21.0


git clone git@gitlab.<myserver.com>:test/spielwiese.git
...
Empfange Objekte: 100% (3/3), Fertig.

Perform changes and commit

git push
...
remote: The gitlab-shell hooks have been migrated to Gitaly, see https://gitlab.com/gitlab-org/gitaly/issues/1226
To gitlab.<myserver.com>:test/spielwiese.git
 ! [remote rejected] master -> master (pre-receive hook declined)

http access works without any issues.

I had a look into gitaly#1226 (closed) and was searching the internet for a solution, but nothing so far. git version is correct and supported, GitLab is installed as Omnibus, ...

Any hints are welcome.