proxmox proxmox 9.0.3 fails to check for updates

I have update to Proxmox 9.0.3 by a clean installation and reissued my license to it. At the moment I get every morning a email notification that apt-get update failed with exit code 100. I tried to clean the lists with rm /var/lib/apt/lists/* -r but that doesn't solve the issue. Does anyone...