FortiGate includes a self-signed default certificate (which is not trusted by a CA, and can't be verified by browsers). This means that if Fortigate is encrypting this connection, it will not be trusted in another browser. To prevent that, you need to install a 3rd party certificate (not sold by Fortinet).
You're accessing the SG-250 (very old switch) via GUI(HTTPS) and its certificate has been expired long time ago. The FGT is just in the middle and checking the certificates (as you configured) coming from the server(SG-250) side and found it invalid. If you don't want to make FGT ignoring invalid certificates, your options are one of these: 1. As Alex says, get a proper certificate signed by one of common CAs and import/install it to the SG-250 [the best option among these]
2. Stop using GUI/HTTPS to manage the SG-250. CLI/SSH or HTTP would be the options.
3. Cisco might have an updated default cert. Ask their community.