SSL-VPN triggering a IPS upd_flood (i think)

Hi y'll

I've got this challenge with a lot of alert emails from my fortigate. It looks like it's triggered trough the DoS IpV4 policy on WAN1. If I read and analize the message i see it's from a customer that uses SSL-VPN connection to our center. It's not just one customer but several that can trigger. I raised the treshold for this DoS IpV4 from 2000 to 3500 and it reduced the amount of mails :) . I can of course raise it even higher or just turn it off but that's not a good solution because I want to understand what happens. I see that it seems to be in connection with the logon (but not 100% sure).

i post the Alert and hope somebody can give me a hint or a solution

Message meets Alert condition

The following intrusion was observed: "udp_flood".

date=2018-06-18 time=12:00:56 devname=FGT92D-prim-SMS devid=FGT92D3G1400xxxx logid="0720018432" type="anomaly" subtype="anomaly" level="alert" vd="root" eventtime=1529316056 severity="critical" srcip=79.160.97.xxx srccountry="Norway" dstip=193.71.1xx.xxx srcintf="wan1" srcintfrole="undefined" sessionid=0 action="clear_session" proto=17 service="udp/10443" count=137 attack="udp_flood" srcport=53797 dstport=10443 attackid=285212772 policyid=1 policytype="DoS-policy" ref="http://www.fortinet.com/ids/VID285212772" msg="anomaly: udp_flood, 3501 > threshold 3500, repeats 137 times" crscore=50 crlevel="critical"

Kind regards