Had the same problem
Try this maybe:
1.Update Elden Ring in steam
2.Open the game in steam and enable all system network settings to online.
3. close the game
4.check if there is any more updates for Elden Ring on steam, update and open the game again on steam.
5.Now open Nucleus Coop (make sure you have the latest version)
6. Download the handler for Elden Ring again and replace with old one/same one (it will give you the option to replace)
7. Now start the game on Nucleus coop
8. Enable all network settings for both instances.
9. Click continue and let both characters rest at a lost Grace.
10. Hope it helps