You can try logging in to your account through a private browser and then try updating your bank account from there. Here's how:
Go to the Banking or Transactions menu.
Select the bank.
Click Update at the upper-right.
If you're able to successfully download recent transactions, you can switch back to the main browser and clear the cache. There are times it causes unusual responses within QuickBooks Online. You can also use other browsers to isolate these kinds of issues.
For more details in handling your bank feeds, check out these articles: