Sophos UTM 9 blocking whatsapp completly

I found in the webfilter log this event:

2017:08:17-17:46:47 sophos httpproxy[5727]: id="0002" severity="info" sys="SecureWeb" sub="http" name="web request blocked" action="block" method="POST" srcip="100.0.0.97" dstip="" user="" group="" ad_domain="" statuscode="502" cached="0" profile="REF_HttProContaInterNetwo6 (Mobiles)" filteraction="REF_HttCffAllowForEit (mobiles )" size="0" request="0xd9145600" url="http://c.whatsapp.net/chat" referer="" error="Received invalid request from client" authtime="0" dnstime="0" cattime="130" avscantime="0" fullreqtime="30038442" device="0" auth="0" ua="Mozilla/5.0 (compatible; WAChat/1.2; +www.whatsapp.com/contact)" exceptions="" category="122" reputation="neutral" categoryname="Instant Messaging"

any help

Thanks