I have had the same problems trying to download a .QBO file from American Express.  I searched their "Help Center" and found this, WHICH WORKED FOR ME!!!!!  Here is a link on the AmEx website for help to download transactions for any date range up to the last 6 billing statements...hope this helps you too!  You may need to first "disable bank feeds" in QuickBooks for the particular credit card account.

https://www.americanexpress.com/us/customer-service/faq.payments-statements-and-balance.download-exp...