For registration with Forticlient for Windows FortiClient registers to the FortiGate using Endpoint Control (EC). In FortiClient 5.0 and 5.2, the default registration port is TCP port 8010. FortiOS 5.0 and 5.2 both listen on TCP port 8010. Starting with FortiClient 5.4, EC registration will use port 8013 by default. To register to FortiOS 5.0 or 5.2, the user must specify port 8010 with the IP address, separated by a colon. For example, <ip_address>:8010. FortiOS 5.4 and later will listen on port 8013. If registering from FortiClient 5.4 and later to FortiOS 5.4 and later, the default ports will match. Specifying the port number with then IP address is then optional.
Solution for older FortiOS 5.2.x: go to config > system > global in the CLI console, setting forticlient-reg-port command will change registration port.