My VS 2015 is already Update 3, which is recommended by MS for nuget to work.
I already deleted Nuget.config and installed Nuget VSIX 3.6.
I can open each URL used in Nuget settings in the browser, and each opens instantaneously over both HTTP or HTTPS.
Still, VS throws this error after a long timeout:
[nuget.org] Failed to retrieve metadata from source 'https://azuresearch-ussc.nuget.org/query?q=&skip=0&take=26&prerelease=false&supportedFramework=.NETFramework,Version=v4.6.1&semVerLevel=2.0.0'. An error occurred while sending the request. The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel. The remote certificate is invalid according to the validation procedure.
The URL in the error above also opens instantaneously in the browser. Its certificate is valid.
I tried running VS as Administrator.
I am concerned about the part "An error occurred while sending the request." but this may be just the way it goes with MS software: an irrelevant bit of text in the error message.
Is there any missing step, such as accepting certificates anywhere?
Suggestions to upgrade to the latest Visual Studio are offtopic. If this was possible, I would not be asking this question.
After asking this question I was successful in using command line nuget.exe v. 3.5 to install specific packages, so this question seems to be related to VS itself.