I recently ran into the problem exactly as described in my own game with my girlfriend. I think I found a work around which I've sent to Pipeworks, but I hope this works for you guys!
If you are experiencing this problem, try going into settings and disabling the client side map. You should be able to enter the hosted game with the map disabled without a problem. You have to play with the map disabled for half an hour or so. Afterwards, re-enabling the map still allowed me to join my girlfriend's game. However, it still loses connection after a few hours or so, so I have to "reset" the behavior by turning the map off and playing with it disabled again for a little bit.
Kind of annoying, but it's been working for the past few days for me. Copied and pasted my bug report below just in case there's any details that helps anyone recognize the problem
ISSUE SUMMARY
A new large world was created and played on with 2 people in multiplayer.
After approximately 20 hours of total play time in the world, the client game began experiencing "lost connection" messages. At first, this occurred about an hour into a session (it may have first occurred during a host side auto-save, but do not recall).
After attempting a reconnect, the host game will automatically drop the client immediately after entering the world. There is no impact in behavior to the host.
TROUBLESHOOTING
1) Software was restarted on both client and host
2) Both switches were power cycled
3) A different client character was able to successfully enter the original host large world. The behavior did not initially present, but after an hour of exploration the persistent lost connection messages returned.
4) The entire inventory of the crashing characters were dumped into a chest in a separate world. When joining the original host large world with an empty inventory, there was no lost connection for 30 minutes. After 30 minutes, the behavior described above would begin occurring again.
5) A new large world was created and hosted by the same host. The client was able to join and play for two hours without a crash. Attempting to return to the original host large world resulted in the same crash.
WORK AROUND
Entering "settings" and setting "Map" to "Disabled" allowed for the original client character (with full inventory) to join world. After playing for an hour or so with the map disabled, exiting and re-enabling the map allows the client character to continue playing for several hours without losing connection. The client will eventually lose connection and restart the problem, but setting the map to "disabled" again appears to "reset" the issue.
OBSERVATIONS
The error reporting usually shows "Lost connection to server." Sometimes the client will instead hang and the host will receive a "Player2 has disconnected" message. The hang went on for about 15 minutes before the software was restarted.
Setting the map to "disabled' causes the client to load much quicker than with the map enabled.
The frequency of the lost connection seems to increase as more of the large world is discovered. The first time a lost connection occurred was after exploring a significant portion of the dungeon for the first time. The 2nd client character did not disconnect until a significant portion of the jungle and dungeon was explored.
The lost connections do seem to sometimes occur during a host auto-save, although I cannot say with certainty that it always occurs during a host auto-save.