HCW fails with error "InternalUrl_Duplicate"

Hi, I'm in the process of setting up an Exchange hybrid configuration. Unfortunately hybrid configuration wizard fails to register the endpoint. MRS is active and Microsoft Remote Connectivity Analyzer does not reveal any issues. The HCW log file has an error message regarding an "InternalUrl_Duplicate".

Here is the full message:

[Client=UX, fn=SendAsync, Thread=16] FINISH Time=612,1ms Results=BadRequest {"error":{"code":"InternalUrl_Duplicate","message":"Internal url 'https://webmail.my-domain.com/' is invalid since it is already in use","innerError":{"date":"2021-10-31T09:56:40","request-id":"e9bec38c-6f56-46da-8465-046c1fdb4eb6","client-request-id":"e9bec38c-6f56-46da-8465-046c1fdb4eb6"}}}

We run Exchange 2013 with latest CU and updates. Exchange web services are published through Azure application proxy.

Any ideas?

Your help is highly appreciated.

Thanks.