Cfr this stackoverflow question:
Helped out a MyGet customer using TFSOnline and a secured NuGet feed + package restore.
Turns out there was a simple typo in the solution-level nuget.config file:
<packageSource> versus <packageSources>
VERY easily overlooked, and apparently no validation happens on the nuget.config file when using it during restore. Wasted quite some time on a silently ignored validation error in the nuget.config file. It'd be nice to warn the user about this in the logs
(or ship the nuget.config xsd with package restore for VS intellisense?).