Cause: Browser cache issues or incompatible browser version. 9.0 dropped support for Internet Explorer.
Fix:
As of Portal 9.0, MFA is required.
Cause: More than 5 failed login attempts in 15 minutes. 9.0 navistar login
Fix: Wait 30 minutes for automatic unlock, or have your Portal Administrator manually unlock it via the Admin Console (under User Management > Unlock User). Cause: Browser cache issues or incompatible browser version