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.
I also read the Xcode Cloud requirements page and everything seems to be correct. The only thing that doesn't work - which should, according to the above linked manual - is AppstoreConnect not accepting the complete GitLab application secret.
So how can i get this to work?