Cannot interact with repository during shell job (error 403)

Hi there,
I’m using the GitLab CI system to build several projects and I would like to automatically tag certain builds in the remote repository.
For this task I created a custom tool that handles this. If the tool could determine the correct version to tag, it pushes this tag to the remote.
In theory this works but as soon as I use this tool in my shell script, I only get 403 error codes.
If I log into the runner’s server and run the script by myself in the console, everything works just fine and the tag gets pushed correctly.

Any hints on how to resolve this issue?
The runner runs Windows 10 Pro.