May I know which specific bank you've connected to QuickBooks Online (QBO)? Also, may I know the error message that you're getting? Any additional information will be much appreciated.
In the meantime, try signing into your QBO using a private browser (incognito). This will help us check if this is a browser-related (cache and cookies) issue.
Here's how:
Press Ctrl + Shift + N (Google Chrome).
Ctrl + Shift + P (Firefox).
Control + Option + P (Safari).
Once logged in, let;'s try to upload your bank transactions again to double-check. Here's how:
In QBO, go to the Banking menu at the left pane to get to the Banking page.
Select the blue tile for the account.
Click the Link account drop-down, then Upload from file.
Tick Browse and locate the file you downloaded from your bank.
Choose Next.
In the QuickBooksaccount drop-down, pick the account you want to upload the transactions into. Then Next.
Follow the onscreen instructions, then Next.
Once done, hit Let's go.
If it's successful, go back to your default browser and perform a clear cache to refresh the system. However, if the issue persists, try using other supported browsers.