Cause: GitHub updated their security requirements on the server side. They no longer support TLS 1.0 and 1.1, only 1.2. Older Git Credential Manager for Windows versions used 1.0.
Solution: Update Git for Windows. Version 2.14.3 and newer include an up-to-date Git Credential Manager for Windows.
Alternatively you can update only the Git Credential Manager for Windows itself. It supports TLS 1.2 from version 1.14.0 onwards.