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The developer of the "SEAMLESS CO-OP" mod for Elden Ring has restricted access to non-Steam versions starting with mod version 1.7.8. This causes a "steam datagram relay error" when attempting to open the world in-game. Additionally, downgrading the mod version and launching ersc_launcher.exe triggers a message: "This version of seamless co-op (1.X.X) is deprecated and requires an update," as the launcher checks the game and mod version through an Internet connection.

Download the latest version here

Solution:

To bypass this issue without disconnecting your PC from the Internet, I created a .bat file that:

1. Adds a firewall rule to block Internet access for eldenring.exe.
2. Launches ersc_launcher.exe.
3. Removes the firewall rule afterward to restore normal Internet access.

The script uses only Windows PowerShell for its operations.

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Instructions:

For Non-Steam Users:

1. Downgrade the mod to version 1.7.7 or below (compatible with DLC, starting from version 1.7.2).
   • Install the mod as you normally would.
2. Download the .bat file from the Releases page.
3. Copy the .bat file to your game root folder (the same location as eldenring.exe).
4. Run the .bat file as Administrator. That's all!

For Steam Users with a Downgraded Version of the Mod:

1. Download the .bat file from the Releases page.
2. Copy the .bat file to your game root folder (the same location as eldenring.exe).
3. Run the .bat file as Administrator. That's all!

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Additional Notes:

• This solution was created primarily for my own convenience and for a friend.
• I am not aware of a guaranteed method to use the latest version of the mod with a non-Steam version of the game, though options like the 'Online-fix' might exist. Feel free to explore that possibility if interested.

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Disclaimer: Use this fix at your own discretion. Blocking and restoring Internet access via PowerShell scripts may have security implications; make sure you understand and are comfortable with the actions performed.