FortiGate Firewall Upgrade Issue – GUI Login Failure in FortiOS 7.6.4

Sharing a real upgrade issue we recently faced.

Scenario:
FortiGate Firewall upgraded to FortiOS 7.6.4
Post-upgrade:

# Unable to login to GUI using admin ID
# CLI access was working properly

Firewall was reachable and traffic was flowing

Troubleshooting: verified all standard checks like:
# Admin credentials and admin profile
# Trusted hosts configuration
# Interface allowaccess (HTTPS/SSH)
# Management interface and routing
# No access restriction from policies

Everything was configured correctly, yet GUI access consistently failed.

Root Cause (Conclusion):
Issue identified as a FortiOS 7.6.4 GUI-related bug
No configuration or access misconfiguration found

Solution:
Decided to upgrade FortiGate from 7.6.4 to 7.6.5
After upgrading to 7.6.5:
# GUI login started working normally
# Admin access restored without any