I have posted a similar post but not exactly the same question.
My understanding is that whe you create a registry end point , the harbor tries to check the remote certifacte so in your case the docker registry
The help online states that the verify remote cert flag should be unchecked when remote registry is using a self signed cert, so it might be normal that the connection test is failing.
In my case, i wanted to know if need to configure the harbor with the CA cert of remote registry and how ;
cna you elaborate on what you did on your habor host