Split-task VDOM mode. Can't ping from FG-traffic

Hi,

I'm using Split-task VDOM mode and I'm filtering traffic through FG-traffic VDOM. The problem is that if I enable Web Filter i get te error : "

...dstport=80 dstintf="internal4" dstintfrole="undefined" proto=6 service="HTTP" hostname="captive.apple.com" profile="g-default" action="blocked" reqtype="direct" url="http://captive.apple.com/" sentbyte=117 rcvdbyte=0 direction="outgoing" msg="A rating error occurs" error="all Fortiguard servers failed to respond"

I read that it seems to be due to the fact I can't ping from FG-traffic VDOM, and in fact I can't ping. From root VDOM I can ping but I don't know how I can ping FG-traffic VDOM through root VDOM, any ideas?

Thanks.