Are you using any mods?
If the host creates a new lobby, can you join then? If so, can the latest save from your current playthrough be loaded?
If you or the host create a new profile and start a new game (without mods), can the other join?
Did either of the other Steam players try joining you via Direct Connection? If they get the same error, can they join using the Steam lobby or invites?
Try verifying local files: in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
If you can join when not using mods, but a new game with the same mods enabled gives the error, try comparing the mod files in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\Mods' folder with the ones in the host's folder (or one of the other players).
You can use a program like DVDSig or ExactFile to scan the folder, swap the files each program creates and verifying against the other's file, to identify any differences in the folders.
DVDSig is tiny (
alternate download), portable and doesn't require an install, while
ExactFile has more features.
If you only have a couple of mods, it would probably be easier to use a program such as
Hash to create a list of checksum values to compare manually (slightly more work comparing, but no file swapping required).
Try resetting your network adapters:
- click Start (or Search in Win 10), type cmd into the search box, right click Command Prompt and select run as administrator.
- type the following command and hit Enter
netsh winsock reset
- when that is done, there should be a message 'Winsock reset completed successfully'
- type exit and hit Enter, then reboot your computer.
For more details, see:
https://iihelp.iinet.net.au/Resetting_Network_Adapter_(Winsock_Reset)Try starting two instances of the game on your system and see if you can join a local co-op game. If the problem is data related, this should work fine (though working wouldn't necessarily mean it is a data problem).
Start the game, switch to windowed mode and lower the game resolution (and maybe check the option to mute the sound volume when the game doesn't have focus), start a multiplayer game, hit Esc, open the Connectivity Menu, optionally disable online multiplayer (if it is enabled) and make sure LAN Connections is enabled (or that can be done in the options before starting a new game).
Next, start the game again from the 'EoCApp.exe' program file (doesn't matter if the first instance is started through Steam or the executable), select Join Online Game in the Campaign section, switch to the LAN tab, rather than Online, and try joining.
