FortiGate VM GUI Immediately Logs Out After Successful Login

I'm facing a strange issue with a new FortiGate VM instance running on Proxmox.

Environment

• Hypervisor: Proxmox VE
• FortiGate: FortiGate VM
• Disk image: fortios.qcow2
• Access: Web GUI over HTTPS
• Version: FortiGate-VM64-KVM v8.0.0.build0167.260420 (GA.F)

Issue

I created a new FortiGate VM using the fortios.qcow2 image.

The VM boots successfully, and I can access the FortiGate GUI login page. I can also enter the admin credentials and authentication is successful.

However, immediately after successful login, I am logged out and redirected back to the login page.

So the behavior is:

FortiGate GUI Login
        ↓
Enter credentials
        ↓
Authentication successful
        ↓
GUI starts loading
        ↓
Immediately logged out
        ↓
Redirected back to Login page

There is no normal session timeout involved because the logout happens immediately after login.

Troubleshooting already attempted

I also tried increasing the administrator timeout:

config system global
    set admintimeout 30
end

However, this did not resolve the issue.

Since the login itself is successful, I'm wondering if this could be related to the GUI session/cookie, HTTPS configuration, system time, VM networking, FortiOS version, or Proxmox virtual hardware configuration.