Raze_Larian: Yes, it should still be possible to join using Direct Connection.
For D:OS 2 you can also try Discord invites, if you use that program.

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 either of you tried joining anyone else?

For D:OS 2, 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.
For D:OS 1 or 2, try creating a new multiplayer game lobby, exit and try joining.

If you are using any mods, try testing in a new profile without any mods enabled.

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.
With the GOG version, in the (optional) Galaxy client, select Divinity: Original Sin 2 in the left column, then click on the More button, and in the Manage menu select 'Verify / Repair'.

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])

Raze_Larian: Yes, it should still be possible to join using Direct Connection.
For D:OS 2 you can also try Discord invites, if you use that program.

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 either of you tried joining anyone else?

For D:OS 2, 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.
For D:OS 1 or 2, try creating a new multiplayer game lobby, exit and try joining.

If you are using any mods, try testing in a new profile without any mods enabled.

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.
With the GOG version, in the (optional) Galaxy client, select Divinity: Original Sin 2 in the left column, then click on the More button, and in the Manage menu select 'Verify / Repair'.

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])