[SR-Users] Kamailio not listening to SIP despite setup OK

Hi all! got a client with 2 Kamailio servers using Pacemaker/Corosync with a VIP. Kamailio is setup with dispatcher, and has been working since 2018. This previous week, client migrated from a Cisco ASA firewall to a Fortinet F5 firewall. One of the Kamailio server is, apparently, working fine, receiving SIP invites and processing them correctly. When moving the VIP to the second Kamailio node, Kamailio is ignoring any SIP message that is not OPTIONS, despite configuration (kamailio.cfg) being exactly the same as in the node #1. Also, note that using SNGrep, we can see clearly that SIP messages are reaching the server, but Kamailio is just not processing any INVITE. In fact, I added an output to log file right at the very beginning of the main route and the line is added to log file ONLY if the SIP message is OPTIONS. When the SIP message is INVITE, nothing is processed on Kamailio's side. The listen parameter is set to 0.0.0.0 port 5060 and advertised IP set to the Public IP. Again, the main route is only : route{ xlog(. ......); } Kamailio version is 5.1.6 (old, I know....) but it has been working since Dec 2018! What could be the issue?! Why only OPTIONS are being , somehow, recognized by Kamailio and all other SIP messages just ignored? Also, to add some more weirdness to the mess, connecting an Asterisk server on the same network, and sending a call to Kamailio node #2, it works!! SIP messages of any kind are processed correctly by Kamailio on node #2. So, why doesn't it work with SIP messages received via Public IP address, even though the messages reach the server correctly (confirmed using SNGrep) ??? I spent 5h this afternoon trying to understand what is wrong, but I'm stuck .... and no clue so far, apart from suspecting of the new Gateway, which I have no access to configurations.... but if OPTIONS are received & processed OK, it wouldn't make sense that received INVITES would be ignored by Kamailio, right? Atenciosamente / Kind Regards / Cordialement / Un saludo, *Sérgio Charrua*