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I would make sure you start a flight on both computers before setting up networking. If it's left on in your preferences when you quit, I would expect it to resume once you start flights. Entering an IPv6 address might not be working so I would also recommend using the drop down to set up networking, rather than manually entering the IP address.
If it is reproducibly dropping the connection, please file a bug here. Include the log.txt for both machines and try to be as specific as possible how you set it up and encounter the not syncing issue.
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