DNS filter - A rating error occurs - all Fortiguard SDNS servers rating timeout

Hello,

I have FG-60E with newest FortiOS 5.6.3. I have following error messages in DNS Query logs:

"all Fortiguard SDNS servers rating timeout"

and sometimes

"DNS query timeout"

FortiGuard Filtering Port: 8888

My internet connection is without firewall limitation from ISP (I hope), and I have public static IP address assigned via PPPoE protocol.

I have paid these licenses:

FortiCare Support IPS AntiVirus Web Filtering Anti-Spam Filtering Mobile Malware

Do you have any ideas what to do? Restart FG didn't help. Errors (due to timeout) occur approx every 1-5 minutes and continuously (for several days).

Thanks for your help :)

Zdenek