IPS logged unnamed Attack description: Failed to Lookup FortiGuard Encyclopedia

Hey all, 

First timer  to this forum here. There's something I want to know but I couldn't find the topic by looking around the web. Recently my FortiGate60E has logged something under IPS which it considers as an "attack" but it has no name. The description only says "Failed to Lookup FortiGuard Encyclopedia" and there is no victim or attacker information. Somehow the Threat level is marked "HIGH" so it warrants some looking into. 

Does anyone have any idea what this means and how can I make them go away? I'm running firmware version 5.6.5. I did not upgrade or downgrade the firmware for some time so this issue should not directly be caused by firmware upgrade/downgrade. 

Thanks in advance.