Fortiguard webfilter services are NOT reachable

Hello.

Recently i'm updated FortiOS to 6.4.4 and now sometimes i see error "Fortiguard webfilter services are NOT reachable"

And when Fortiguard services are NOT reachable users don't have internet access.

When i'm disabled all inspection (webfilter etc.) i'm don't receive any error about access to Fortiguard services. If turn it back then periodic receive error "are NOT reachable"

Before upgrade from 6.0.10 to 6.4.4 (follow recommended upgrade path) everything work fine. 

ISP connection work fine.

Any ideas how fix it?

I'm use FG VM on Hyper-v.