A few months ago I solved a problem with a site-to-site VPN tunnel between a SonicWall NSA appliance and an Azure VM running Barracuda NextGen Firewall.
This VPN tunnel would go down once every 2-3 weeks, and only be restored if I manually re-initiated it from the Barracuda side.
I had intended on saving a draft of this post with details on the error I found in the Barracuda log, but apparently failed to do so.
So the short answer is make sure that “Enable Keepalives” is turned on, on the SonicWall side of the tunnel. This has brought stability to the VPN long-term.