Heartbeat device(interface) down

hello to evrybody. I have a cluster made of 2 fortigate. sometimes I get from logs this error from both appliance: Heartbeat device (interface) down - neighbor info lost I changed the cables that connect the appliances. But I get the same errors. Does anyone know what are the reasons of this errors? maybe one port of the appliance has problem? how could I get more information? I also checked the interface messages but there are no errors. thanks.