Hello, I’m Virginia an independent advisor with 20 years of expertise in fixing my own & friends’ PC problems. Which Windows 10 build are you running - 1909, 2004, 20H2 or 21H1?
I don't think I will be able to move on without fixing the problem in Assassin's Creed 3. And DigitalAssassin I am sorry to disappoint you with this information.. But I promise you and everyone else, I won't cheat ever again with those Save Files things.. After all DigitalAssassin? Nothing is True, Everything is permitted.. Oh..
Unfortunately the servers are Activision's responsibility & it's well known they don't really care about fixing anything on their servers for the older COD games. Only thing you can really do is if you suspect something isn't right quit to the dash straight away (press the guide button on your controller & quit to the xbox dash).
Clean boot disclaimer: Please go through the section: How to reset the computer to start as usual after troubleshooting with clean boot of the Kb article to boot the computer in normal startup after fixing the issue. Disable Antivirus disclaimer: Antivirus software can help protect your computer against viruses and other security threats.
The Dell people do not know how to fix this an they have no interest (no compelling reason) in fixing it. What you should do is create a bootable XP Recovery Console CD (no XP media required) boot into the XP Recovery Console and run chkdsk /r and then fixboot.