Диагностика проблем с VPN начинается с включения дебага на шлюзе:
Sk63560: How to run complete VPN debug on Security Gateway to troubleshoot VPN issues?
sk33327 - How to generate a valid VPN debug, IKE debug and FW Monitor?
sk89940: How to debug VPND daemon
skI4326 - Enabling IKE and VPN debugging
sk34467: Debugging Site-to-Site VPN: