I am having this issue as well following the The client transfer between ePO servers guide for MDE 713. My ePO is 5.3.1 and the extensions are installed. Do I need to install the scripts to ePO? I have a ticket open with ePO and DE team and we are all stumped.
I'm guessing that you have two (2) ePO servers; one old ePO server and one new ePO server. Are they both running the same version of ePO?
I would personally advise that trying to figure out the syntax for listing the registered servers using Web-API is a waste of time, unless you're going to be automating it for some reason. I'd just check the list of registered servers in the ePO web interface if I were you, and only use Web-API if you really really have to, just to save yourself never-ending frustration. The list of registered servers is unlikely to change regularly unless you have colleagues changing things unexpectedly in ePO. If you do, you have other issues to deal with first before worrying about ePO Web-API syntax.
As an example of how specific the Web-API commands can be, on an old 4.6 server with an ancient version of Drive Encryption, I execute https://server:8443/remote/core.help (I personally use curl for windows). It returns all the available commands on that server. Even though some eeadmin commands are available, the eeadmin.listRegisteredServers command is not one of them, most likely because the ancient version of DE in use never had that command available in the Web-API. So as Eric Vaughn said,it depends what you're running, and which ePO server you're trying to run it on (the old one or the new one) and which version of ePO is on the old one and the new one.
Please provide more information on your scenario and we might be able to assist.
Not sure if you can send private messages on this system - I had a look and couldn't find any private message function. You can't just give a basic overview for us? Nobody wants to know your customer's server names or IP addresses, just software version details on each ePO server and the scenario in which you're trying to perform migrations.