It works for certain URL's. But does not work for some.
We know the sources are working as other users can access.
(I myself added this from ArcGIS Explorer, from another machine. And objects show up on Map.)
My question is - is there a way to provide troubleshooting information?
So far, my experience with this has been 'hit and miss'.
The failure point is - it does not show me folder (as I expect) to select after connecting to the provided URL.
I do not get any clear message, simply a screen without folder name i.e. nothing for me to select.
Steps - Add content. GIS services.
Provide Server URL, authentication (username, password).
Adds a new entry "arcgis on URL provided". Shows green.
I select and click "Add".
Next screen ("Add Data") does not show me folder name contrary to my expectation.
When I try again, the previous entry appears in red and the number incremented.
"Red" entries keep on adding up as I try again and again.
Eventually, I use ArcCatalog to clean things up.
ArcGIS 10.1 has been formally/completely retired since 01/01/2018. Troubleshooting retired software is difficult because the version gap between what most users are using and the old software is so big. It could be your ArcGIS Server is a newer version and there is some incompatibility between an older client and newer server.
Can you upgrade to a newer version and test again?