I suddenly started receiving this error when attempting to push changes from VS2017 to a VSTS Git repository. This functionality had worked the day before.
I checked my git.log file and saw a different exception :-
19:43:57.116665 ...zureAuthority.cs:184 trace: [ValidateCredentials] server returned: 'Unable to connect to the remote server.
I downloaded the latest Git CredentialManager source from Gits Credential Manager repo and debugged it.
Once authenticated, the following exception occurred :-
No connection could be made because the target machine actively refused it 127.0.0.1:8888
I then realised that I had recently setup Fiddler to act as a proxy for all services as per the article capturing-traffic-from-.net-services-with-fiddler
Once I ran Fiddler, I was able to successfully connect.