This indicates an attack attempt to exploit an Information Disclosure Vulnerability in FortiOS.
The vulnerability is due to an error in the vulnerable application when handling a malicious request. An unauthenticated attacker can exploit this to access sensitive information on the affected machine via a crafted request.
FortiOS 5.6.3 to 5.6.7 with SSL VPN Service enabled
FortiOS 6.0.0 to 6.0.4 with SSL VPN Service enabled
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Refer to the vendor's advisory for updates: