Another way to resolve this is this time very simple: if you are seeing this error in the MPOS UI, and you go to the Event Viewer and there are no events from a Retail Server source, then for sure you have a simpler issue.
This is the Retail Server URL you have entered is wrong, for sure. Just copy it and browse it in a web browser. If valid it should let you know that it wants to download a v1 file, but this will not happen. Just correct this and it will work.