Added a private VS extension gallery as described in
https://msdn.microsoft.com/en-us/library/hh266746.aspx
using an atom feed as described in
https://msdn.microsoft.com/en-us/library/hh266717.aspx
And got this error message:
"A connection to the server could not be established because the following error(s) occurred:
Could not determine the protocol used for this gallery. The URL must point to an Atom Feed or SharePoint List configured for hosting Visual Studio Extensions.
Please click here to retry the request."
This guy had the same problem but didn't bother to post his solution (and apparently you can't reply once it's "solved"), so here it goes: