website certificate error after register forticlient

hmm I guess you are not using split tunneling. So all internet traffic from the client goes via the vpn and through the FortiGate. I also guess you have ssl inspection enabled on your internet policy. 

In this case the FortiGate will decrypt https connections, inspect them and then recrypt them usig its own certficate (since it of course does not have the private key of the original cert). So client will get https with Fortinet self signed certificate (which might also be invalid by timestamp).