Error 0x8000ffff

So, according to the pop up for this, it's a "catastrophic failure". It might be a Windows system corruption. Sea of Thieves relies on some system files for Xbox Live, so it can be affected when other games are not.

First thing to try though is signing out of the Microsoft Store and Xbox App and then signing back into both.

Then try resetting the Micosoft Store by doing Win+R and entering:

wsreset.exe

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If neither of those work, try running SFC and DISM scans.

To run an System File Checker scan, search in Windows for "Command Prompt", right-click on it and "Run as Administrator". Then type and enter:

sfc /scannow

Wait for the scan to finish (it may take a couple of minutes).

To run DISM enter:

DISM.exe /Online /Cleanup-image /checkhealth

followed by

DISM.exe /Online /Cleanup-image /restorehealth

Again, wait for the scan to finish. It will take a lot longer than SFC.

You can run SFC again after this, if you want to be sure. It's actually debatable whether you should run SFC first or after. I tend to just be thorough and run SFC, DISM, SFC.