I followed Apple's instructions
When i come to the step where one should copy the auto-generated secret from the newly created GitLab Application - i can't proceed without an error.
The auto-generated secret from the GitLab application is 64 characters long + a prefix: "gloas-".
So it is actually 70 characters long.
The AppstoreConnect website form that wants this secret doesn't accept it, saying, it is too long and also in the wrong format. If i enter the secret without the prefix, the form accepts the value - but the authentication attempt that follows afterwards fails.
I tried the whole process with a fresh Xcode project. It still failed - so i'm pretty sure that the project is fine and the problem is most likely the GitLab Application secret.
So how can i get this to work?
Update
I accepted the answer from Jessy Wesson since it links to a workaround in my Apple Dev Forum thread and an accepted answer may guide others better. However - it is only a workaround and not the solution, but - we can assume that there is no solution right now and only Apple can resolve this properly, so the workaround seems to be as good as it gets.
This issue has been reported to Apple. If it helps, there is a workaround mentioned in the following thread on Apple's Dev Forums here: https://developer.apple.com/forums/thread/738513