Server App Version 5.3.1 won't take password
Make sure you are using the latest version of Server that can run on your OS. You can try to repromote Server, by closing the app, moving it (the Server.app in /Applications) to the trash and waiting for the "Server Removal Detected" dialog to come up. After that you move it back to /Applications and launch it. It should ask you if you want to update Server on this machine. Update and see if that solves your problem.