Seems like nuget.org removed support for default TLS version (1.1).
Run this command in NuGet command control (PM) to use version 1.2:
[Net.ServicePointManager]::SecurityProtocol=[Net.ServicePointManager]::SecurityProtocol-bOR [Net.SecurityProtocolType]::Tls12
However, it seems like a non-permanent solution, so after restarting Visual Studio, you need to run command again. Too bad.
Please notify if/when you find permanent solution!