I am having the same issue. I have a service that was working fine about 1 month ago, now it stopped working. I checked the Pro project and all the layers have exclamation marks. When I try to set the data source again, it gives me the "Layer's data source could not be changed". The layers work, they have been working fine. I can add them to any other project. Why would it break all of the sudden and not let it repair?
