If you get this message, it simply means that some transactions have been presented in the new month. The date shown in the Bank Reconciliation screen under Default Present Date is always yesterday's date. So if Saturday is the 31st of December, then Sunday is the 1st of January. When you go to do month-end on Monday, it will show Sunday's date and everything you present down will be in January.
To Fix: Go into the Presented date, scroll to the end and find the date in the wrong month, go into it and tick all the items and unpresent, then go and change the default present date (left-hand side) back to the last day of the month and re-present the items down. (Double-check that totals are the same and you still reconcile)
Note: The actual date of the transaction is not important here and will not cause this error