Yes, it is possible to make a user connection to this AcrGIS Server without any problems, without credentials needed (see the image). It is just a basic, "open", ESRI´s sample server, I have put it into my code just to show the error, just because it is easily accessible... [ATTACH=CONFIG]23992[/ATTACH]
When I try to duplicate your internet connection to this server I am prompted with a dialog box that asks for Proxy Credentials. I can not get past this prompt with either my network credentials or AGOL credentials.
The below thread shows another user who was unable to use the ListMapServices method in python. It is interesting how ESRI's response did not show this person the correct syntax to actually get this method to work. I have also reproduced your error even on an internal server so it seems something is missing from the ESRI documentation.
I was finally able to make a user connection to http://sampleserver1.arcgisonline.com/arcgis/services from ArcCatalog on my system. There might have been organization network issues (that seemed unrelated) which caused a disruption in being able to connect to ESRI's public server.
Here is the response I received from ESRI after I opened an incident:
"Thanks very much for the links and details. I was able to call ListMapServices successfully on 10.0 for one server at the office but not for another one, and I don't know what the difference is in their configuration. This function, however, is no longer supported at 10.1: http://resources.arcgis.com/en/help/main/10.1/index.html#//00s30000004s000000
Have you tried running Fiddler or other network analysis tool to see if any other details can be obtained? http://fiddler2.com/ There could be some restriction due to permissions, group policy, or other network settings."
I tried running the simple sample script from a machine remote from the AGS server that had ArcMap v10.0 sp4 installed on it. I received the same error message as you. I then tried running the simple script on the AGS server itself and it worked as expected.
I then tried to create an Admin connection to the AGS server from the remote machine using ArcCatalog. This connection failed as per this document:
It seems that this is a problem with a Windows 2008 Server. If this is the same scenario for you, you could try upgrading the machine you are running the script from to Windows 7.