marblebeard error in sea of thieves

Uninstall and reinstall the game

As mentioned this is a NAT issue. If you've currently been playing the game without issue you may want to reset the router and clear the MAC address ion the console

I have read experiences where the Marblebeard error came up as result of a Strict NAT. Take a look at your network information via the Xbox Settings app and check to see what results you have.

https://support.xbox.com/en-US/xbox-one/networking/network-settings

You can also submit a support request for the game here:
https://support.seaofthieves.com/hc/en-gb/requests/new

Hey Xbox Fan,

Thank you for reaching out. If you haven't already, run a connection test by doing the folowing:

1. Press the Xbox button
   to open the guide.
2. Select Settings.
3. Select All Settings.
4. Select Network.
5. Select Network settings. Any known outages will appear in the middle of the screen.
6. On the right side of the Network settings screen, select
   Test multiplayer connection.

In the case and error message appears, please follow the corresponding troubleshooting on our

Connection Solution page. For additional help. we encourage you to reach out to
Sea of Thieves support.

And it does say Open NAT. Thats an interesting one. Perhaps theres more information associated with that code than I have been able to discern. Perhaps try submitting a support request on the Sea Of Thieves site from the link posted earlier. Their team
may have more ideas on how to resolve the issue.