Are you shutting down all non-essential programs before starting the game, especially anti-virus and anything network related (utility or monitoring programs, etc)?
Have you checked your firewall settings? Many routers have built in firewalls, which could block inbound connections from the game.

Have you tried playing with any other Steam users, or your friends any other GOG users?

Have you tried hosting/joining an Arena match?

Try going into Arena mode, create an arena, open the Connectivity menu and check the settings, then exit to the main menu and try joining in the lobby. Alternately, create a multiplayer game lobby, exit and try joining.

You can try joining by LAN, if you don't mind using VPN software (such as Hamachi or Tunngle).
If you use Discord, those invites may also be an option.

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:
[url=https://iihelp.iinet.net.au/Resetting_Network_Adapter_(Winsock_Reset]https://iihelp.iinet.net.au/Resetting_Network_Adapter_(Winsock_Reset[/url])

If you can not join other people, either, try starting 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/Galaxy or the executable), select Join Online Game in the Campaign section, switch to the LAN tab, rather than Online, and try joining.

Maybe it is related to ISP / internet networking problems, or throttling? It isn't a system setting or configuration issue, at least.

It shouldn't be. There have not been any compatibility issues found so far during testing for the next game update, at least.

This has not been reproducible with QA testing cross platform multiplayer.

If the two Steam users or two GOG users are both in the same region, or both using the same ISP, etc, it could be a network issue. Happening in pairs makes it less likely to be a system issue.

Try verifying local files, if you have not already done so: in Galaxy 2.0, select the game, then the settings icon at the top right (beside the Play button) and under Manage Installation select 'Verify / Repair'.
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 Files...' button.

Can anyone try switching between a wired and wifi connection, if available, to see if that makes a difference?
If possible, testing with a mobile phone's wifi hotspot feature could help confirm if it is a network.


Please email supportdos2@larian.com with a dxdiag report (WinKey-R, type in dxdiag and hit enter, then when it finishes loading click on the 'Save All Information...' button and save the report somewhere handy), preferably for all systems, or at least one of each platform.
If you can do a couple of connection tests, that will hopefully help identify the cause of the problem.
First, each check the respective '..\Divinity Original Sin 2\DefEd\bin' folders for the network.log file and rename it. Next have the two GOG users start a new multiplayer game and join, and the same with the two Steam users, exit, rename the network.log files, then have one GOG user and one Steam user host a new game and have the other two people try to join the opposite platform game and, after getting the error, exit and rename the files again, and email with all copies of the network.log files for each of you (as well as the gold.log file in the same folder, if it is not empty).