Can't clone Git projects

Hi,
I installed GitLab EE 17.5.2 on my vServer in a docker container. I can access the system via web interface without any problems.
I can commit and pull in existing projects on my PC. If I want to clone a new project on my pc with

git clone https://gl.xxx.de/php/project.git

I will get the error message

The requested URL returned error: 500

Do you know where the problem is?
Thank you for your help.
BR
martin