That's a tough one to figure out. Some of the big "gotchas" are:
- Previously Reconciled transaction was deleted
- Previously Reconciled transaction was changed
- Big possibility that in Dec. 2021 they made an entry into Jan 2021 when they meant 2022
- And amount was modified
- Are you over or under? So are you looking for a Payment or Deposit?
You get the idea. Maybe start with sorting the Register by Amount and see if you can find a transaction for the same amount. Next, start a Reconciliation (doesn't matter the numbers are wrong) and just look to see if you find anything pending that's dated Jan 2021 or any other transaction out of wack. Also just look through Dec and Jan to see if you see anything out of wack.
Next, I'd take a chance that something changed in Dec. OR to confirm the Nov Reconciliation was correct, UnReconcile December and do it again. Hopefully the ending number in Nov will match the beginning number for Dec. If not, then you've got a bigger issue and the prior Bookkeeper may have just been checking things off vs. actually reconciling the accounts.
From there I'd check with the Accountant. They may be okay with just an entry to fix it (pending the amount). If not, it sounds like a fair amount of time will be involved to UnReconcile back to where you find there error and then move forward again.
Hope you'll find an obvious error.