The Log shows "TCP packet received on non-existent/closed connection" message.
03/26/2020 28 18401
DESCRIPTION: This Log message is displayed when the SonicWall firewall receives a TCP packet after a particular connection was closed.
This is a generic error message as there are a number of root causes that can create this error. For example: A packet was delayed in transition for some reason and reaches the SonicWall after the connection was closed. The SonicWall Firewall drops the packet and logs the message.
Note: This log message is categorized under Network Debug. In OS Enhanced it is under the Legacy Category.
Uncheck Network Debug, Denied LAN IP and VPN Stats Log Categories (On OS Enhanced: uncheck Legacy Category).
Increase the Inactivity timeout of the rules on the SonicWall.
VPN tunnels: You may need to increase activity timeout on the LAN to VPN as well as the VPN to LAN rule to avoid timeout conditions.
Make sure the upstream device, source and destination computers connecting to each other do not have latency.
Upgrade Firmware to the latest version. You may wish to contact technical support to determine whether or not a firmware upgrade is an appropriate action for your environment and issue.
You can check the most recent firmware versions available for your product through your MySonicWall account.