Hi Sarif,
This message means the FortiGate AntiVirus detected a virus coming from your machine. This means at minimum some UTM is enabled, but more importantly your machine is likely infected. There' s a small chance it' s a false positive, but usually that is a big warning sign.
Once you have determined your machine is free of virus (use a couple of scanning tools to be sure), try again. If you continue to face this issue, please open a TAC case. Include the following:
1. Backup of system config
2. Diag debug report / exe tac report
3. Packet capture from your machine and/or on the FortiGate when the issue is occuring.
I am confident they will get you fixed, and probably with a lot less back and forth questions than I would have since they could see your entire config.
Cheers!
--
Sean Toomey, CISSP FCNSP
Consulting Security Engineer (CSE)
FORTINET— High Performance Network Security