Error on importing QIF file: No transactions later than accounts start date

We've been happily importing bank transactions using the QIF file import until November, when we got the error:

"Your bank statement was imported successfully but contained no transactions later than your accounts start date."

That doesn't sound right ae QIF file contains transactions for October 2020, and the accounts start date under Financial Settings is 01/12/2015.

The import worked fine with the September 2020 QIF file, and the October QIF file looks to be in the same format as the September file.

Can anyone advise how to resolve this?