"wrong project_slug or project_id" after 404 response from backstage backend

[JuliusMu]

Hi,
I try to get this plugin up and running. Locally everything just works fine and I can access my own gitlab instance without any issues. But after deployment I get a 404 response from this url:
https://backstage.our.domain/api/gitlab/rest/gitlab.our.domain/projects/mygroup%2Fmyproject?
I already tried the authentication via the gitlab integration token and via oauth. Both have the same result.
The Backstage Gitlab Integration for importing the entities into the catalog works just fine.
Do you have any ideas what could be the issue?