While on K2 5.0, K2 Windows STS was disabled through K2 Management > Claims > Issuers. However, this setting was changed after upgrading to K2 5.1.
Before upgrade to K2 Five 5.1
On K2 Management > Claims > Issuers, K2 Windows STS is disabled and K2 Form STS is enabled:
Linked Issuers to K2 Form STS:
After upgrade to K2 FIVE 5.1, K2 Windows STS is enabled:
This issue has been logged as a bug to be fixed in a future release.