Thanks for taking the time to post about this Walmart behavior you are running into. Having this knowledge in the community on hand will be tremendously helpful.
This error message, (remote server - (520) Unknown) is an error in the Walmart Connection side of the integration. Very often these are resolved by either Walmart directly (most likely) or by reconfiguring the Walmart - ShipStation integration from your store settings area.
If you reconfigure the connection, and continue to see this behavior, I would recommend checking in with our Support Team to see if others are also getting this message and its widespread, or to otherwise continue a more direct troubleshooting investigation.