Hi Reto,
This issue reminds me very much of this other one: Merge request decoration: Remote host terminated the handshake
Can you double check that PR Decoration is enabled at project level in SonarQube? Go to Project Settings > General Settings > Pull Request Decoration and select your Configuration name . If you’re using GitLab CI, you can leave Project ID blank, as it will be auto-detected.
Thanks, regards,
Daniel