In your replay you said it didn't change anything and that it was still the same error message, that's not the same error from you initial report. seams something did change, no?
when it tries to test the connection it will try to upload a file to the site:
then access that file using:
the unidentified error message means that it cannot access that file using the address specified in the URL prefix.
it amy not work till there is a domain name.