Problems acquiring GitLab EE AccessToken

Hi!

We got a new GitLab server (v.18.5.2 EE) that should replace our current instance and I was trying to setup the VCS connection.

1. I successfully created an application in GitLab (trusted = true) 
2. I was also able to create a connection in TeamCity “root” project to the Gitlab-Application
3. when I try to generate a token using the newly added connection the popup with the callback link (http://[IP]/oauth/gitlab/accessToken.htm) generates an error “Error while obtaining GitLab access token”

The same procedure is working flawlessly with our old GitLab instance (v.15.5 EE) that ist still running as long the deployment workflow is not working.

Any hints what I could do to finally be able to generate the token?