Do you get the error if you quit and restart the game, rather than return to the main menu?
Are you using the Party Size Evolved mod (Steam guide
5+ Player Multiplayer Coop Guide) or any mods that require extra files be placed in the install folder?
Long load times when joining can be caused by such mods, if one or more people hadn't done the extra install step, so the mod configurations were different between players.
For single player:
Are you shutting down all non-essential programs before starting the game, especially anti-virus and any backup programs that may be monitoring the My Documents folder?
Try exiting out of the Steam client, if applicable, and starting the game directly from the '..\SteamApps\common\Divinity Original Sin 2\DefEd\bin\EoCApp.exe' program file, by right clicking and running as administrator.
If applicable, disable Steam cloud support either globally (in the client click on the Steam menu and select Settings, and then Cloud) or just for this game (in the library right click Divinity: Original Sin 2 and select Properties, then switch to the Updates tab and check the Steam Cloud section).
Alternately, exit out of the Steam client and just start the game directly from the executable when required.
Next, try browsing to the '..\Documents\Larian Studios' folder and rename the 'Divinity Original Sin 2 Definitive Edition' subfolder.
This folder contains the saved games, configuration files and a level cache folder. Deleting or renaming it will get the game to recreate it on startup; playing the game from a different Windows user account would effectively do the same thing. With Steam running and cloud support enabled, the client would just download the cloud copy of your existing profile.
After that, start D:OS 2 DE, create a new profile, start a new game, save/load and exit back to the main menu. If that works, try copying the pak files from the Mods folder in the renamed D:OS 2 DE folder into the new Mods folder, and try a new game with the same mods mods enabled that you are using now. If that works, exit and then copy a couple saves from the renamed folder into the newly created profile's ..\Savegames\Story folder to test, and if you can load them and return to the main menu, move the rest of the saves over.
If loading a save and returning to the main menu still gives this error, delete the new My Documents D:OS 2 DE folder and rename the original back again.
If a new game in a new profile also gives this error:
Try creating a new Windows administrator user account (either with the My Documents folder in the default location, or on a different drive, if available), switch to that account and try starting the game from there, directly from the executable.
Try doing a clean boot and then test the game. Click Start, or hit WinKey-R, type in msconfig and hit enter; in the General tab, click Selective Startup, uncheck Load startup items (if required) and leave Load system services and Use original boot configuration options checked. Next, click on the Services tab, check the box to Hide all Microsoft services, then click the Disable All button (maybe make a note of which are currently enabled/disabled), then click OK and reboot the computer.
Run msconfig again to switch back to the normal boot configuration.
If that troubleshooting does not help with multiplayer, as well:
If you check your Mods folder while joining, is there a pak file being downloaded there? (..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\Mods)
There was a known issue which could cause some of the earlier patch pak files to be transferred to the client when joining. That issue should be fixed, but this can also be triggered by mods (such as the party size mod).
Have you tried cancelling and trying to re-join? With the original issue a couple of people reported that it was intermittent, so re-joining could avoid the download.
If you and the host create a new profile and the host starts a new game without mods, do you have any issues joining?
Try exiting out of the Steam client, if applicable, and starting the game directly from the '..\SteamApps\common\Divinity Original Sin 2\DefEd\bin\EoCApp.exe' program file, maybe by right clicking and running as administrator, and join by Direct Connection (the host can get a Server ID in the Connectivity Menu after starting a new game, or loading a save) or LAN (but if you are not on the same local network it would require VPN software, such as Hamachi or Tunngle).
One person for the original issue reported that starting the game from the executable avoided the download (they didn't specify if they needed to exit out of Steam).
If this 'install' when joining just happens when using mods, try narrowing down which mod or mods trigger the issue. Move half the mods out of the Mods folder, and have the host do the same, start the game from the executable and check joining. If that is fine, move back half the moved mods, otherwise remove half of the remining mods, and continue like that. Even with a lot of mods that can narrow down the problem relatively quickly; with few mods, it may be just as easy to check a couple at a time.
It may also be useful to double check that the mods themselves are identical.
If you and the host each use a program like DVDSig or ExactFile to scan your mods folder, and swap the resulting files to verify, it would identify any differences between the folders (missing, outdated or corrupt mods). Unsubscribing and re-subscribing to mods where there are differences should presumably make sure everything is up to date, or redownloading from Nexus Mods.
DVDSig is tiny (
alternate download), portable and doesn't require an install, while
ExactFile has more features.
Alternately, you could use a program such as
Hash to create a list of checksum values to compare manually (slightly more work comparing, but no file swapping).
