I have the same problem. When I access the Web for the HA IP of FortiGate and then click the CLI icon, I get this message. When I access the management IP for any one of them, it works well. The HA and management IP addresses are in same subnet.
The problem with the HA FortiGate that does not has a management IP on the interface.
Cell : +201001615878
E-mail : firstname.lastname@example.org