We are also experiencing the same issue with our online multiplayer farm. The host can load the farm, but whenever another player tries to join, the host gets the error "the software was closed because an error occurred."
Switch and SDV software is up to date. Please help - we've tried everything we can think (including having the host sleep and move to the next day.
Just as an update - we can join another saved farm without a problem, so I'm wondering if it's an error with the saved file. The last time we played, there was an error where it took a long time to save. Is there any way to fix the saved file? We've put so much time into the farm...