This is an issue with your stored credentials in the system credential cache. You probably have the config variable 'credential.helper' set to either wincred or winstore and it is failing to clear it. If you start the Control Panel and launch the Credential Manager applet then look for items in the generic credentials section labelled "git:https://github.com". If you delete these, then the will be recreated next time but the credential helper utility will ask you for your new credentials.