Mirroring from private gitlab to gitlab.com using deploy tokens succeeds initially but fails with bogus error later

Summary

I have a repository in a private gitlab (CE) that I want to mirror for CI and pages on gitlab.com. Gitlab.com successfully creates an initial mirror but fails to keep the repositories in sync.

Steps to reproduce

I created a gitlab.com project as "CI/CD from external repo", entered the token credentials.

What is the current bug behavior?

It successfully creates a mirror of the private repository but does not mirror any subsequent changes. It also reports this bogus error message in the settings tab (see attachment).

What is the expected correct behavior?

Changes should be mirrored or a useful error message should be displayed.

Possible fixes

no idea