"Add Device" wizard in FortiManager fails

I try to add a FortiGate(FortiOS v6.0.4) to FortiManager(FortiOS v6.0.4) using FortiManager "Add Device" wizard. The FortiGate I try to add is with the factory-default configuration and it has received the network parameters from the DHCP server. In addition, the FortiGate has fgfm(FortiManager access) enabled:

config system interface edit "port1" set vdom "root" set mode dhcp set allowaccess ping https ssh http fgfm set type physical set snmp-index 1 next end

According to "show system interface ?" the port1 has the public IP address and that's the one I'm using in FortiManager. When I insert the IP address, username(admin) and leave the password empty as by default there is no password, then I do see that TCP three-way handshake to FortiGate TCP port 514 succeeds and FortiGate and FortiManager communicate with each other. However, after few seconds, the FortiManager reports that "Probe failed". There is nothing useful in the FortiManager logs.

What might cause such behavior?