FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article describes how to fix the problem wherein Windows 7 users encounter the FortiClient message 'The server you want to connect to requests identification, please choose a certificate and try again.(-5)' when connecting to SSLVPN.
In this example, the FortiGate was using 8192 DH Params on its settings:
# config sys global
Unsetting DH-params will revert its value to the default one which is 2048.
PS: Note however that this article is one of the possibilities in relation to this error as it could also mean other things and points to other problems.