Good job on successfully importing your transactions last week! The Bank Upload feature in QuickBooks Online helps you bring over multiple transactions at once. There are a couple of reasons why an OFX or QFX format can be invalid. I'll show you how to handle this in your account.
Thanks for joining in on this thread and letting us know you're seeing the same issue when trying to use an OFX or QFX file to upload your banking transactions. It's important to me that you're able to move forward with entering your transactions into QuickBooks Online, so I encourage you to reach out to support as well to work with one of our team members.
Doing that will give you the opportunity to share what you've tried already as well as your account details in a secure way in case we need to let our engineers know about the issue. Here are the updated contact details.
Schedule a Callback or start a Chat: click (?)Help in the upper right > type and enter "Contact support" into the QB Assistant > click Contact Us > explain your situation > click Let's Talk > choose Get a callback or Start a Chat. Support is available from Monday to Friday between 9 a.m. and 8 p.m. ET.
I noticed that you've also visited the France QuickBooks Community before. If you're working with that version, please use the following contact details instead since what I've noted above is for QuickBooks Online Canada. Here's the article for how to get in touch with our support team for the France version of QuickBooks Online: Soutien technique de QuickBooks