[NEBULA] SitetoSite VPN Disconnecting every 30 Seconds

Site to Site VPN between NSG100 and USG60 was working fine for several months, starting last night it began disconnecting/reconnecting. Monitoring from the USG60 side shows it disconnects after 30 seconds, rebuilds the tunnel then disconnects again. No changes had been made prior to this issue. In Troubleshooting we have updated firmware on USG60, rebooted both devices, tore down and rebuilt tunnel multiple times, but the problem stays the same. Help!