FortiGate 30E - Unable to contact server

I have a VPN established to one of our client's sites as I need to make alterations to their 30E's settings. When I enter my credentials into the login box that appears when I browse to the IP address, I get the message 'Unable to contact server' (image attached). However, I can successfully SSH to the device using the same credentials. 

I have rebooted the device. It is version 'FortiGate-30E v6.0.5,build0268,190507 (GA)'.