Experiencing same issue. All on 1.6.9.37. We are sitting next to each other with two Switches. A friend is connecting from across the country. Wife boots the farm with co-op, online communication. Friend can join just fine. When I join, the game shows the same error. I never make it past the loading screen. We are on the same Wi-Fi network. We replicated this 4x. First time was when wife booted world and I tried to join. Crash. Then we repeated it again not understanding what happened. Crash. Then I stayed out and distant friend connected. Friend connected and they played a whole day, no problems. I uninstalled my game and redownload fresh. Also did a hard restart on Switch. On the next Stardew day, I attempted to join. Crash.
To check whether there is an issue with two Switches connecting through online but originating on the same Wi-Fi network, I disconnected my Pixel cell phone from the home WiFi and hotspotted the cellular connection. Connected the Switch to the hotspot and attempted to join using the separate connection.
This WORKED!
Same connection issue?