Reset NuGet credentials in VS2017

As SayusiAndo guessed, the credentials were stored in Control Panel -> Credential Manager. After removing of which everything works fine.

UPDATE: Two gotchas (as @Jurri mentioned in the comments):

1. There may be two credentials to delete (Windows and Generic). The generic one, in my case, had "VSCredentials_" prefix.

2. Restart VS.

I ran into a situation where the NuGet credentials I entered into Visual Studio were not in my Credential Manager. Deleting the package source in Visual Studio, closing Visual Studio, reopening Visual Studio, and recreating the package source allowed me to re-enter my credentials.

Hence, if the credentials don't exist in the Credential Manager, it appears deleting the package source and closing Visual Studio may be sufficient to cause Visual Studio to forget the saved credentials.