The latest release of Visual Studio 16.10.4 has some changes that affects authentication, from what I can see at least.
Despite logging in and out in the dev.azure.com, authentication failed with the message
"Git failed with a fatal error. Authentication failed for https://dev.azure.com/..."
The issue was the Git Global Settings.
The Credential Helper was set to 'Unset'
✅The solution was to change it to GCM for Windows (and redo the login process again)