fork synching by using mirroring repositories fails

I am trying to learn how to work with GitLab. I have a repository and various branches and all that works fine. I also created a forked repository, which also works fine. Now, somehow, I automatically want to get an updated forked repository whenever some change occurs in the upstream main repository.
For this I used “Settings -> Repository -> Mirroring Repositories”. I made both a Push and a Pull “mirrored repositories”. However, whenever I change something in the Upstream, I do not get these changes in the Fork. Even when I click on the “Update” button on the side of the mirrors (see image), I only get an error.

The error I get is: 2:Fetching remote upstream failed: fatal: unable to access '": The requested URL returned error: 301

Best regards,