I'd be glad to assist you today, @henningrst-gmail.
Let's update your bank account in QuickBooks to refresh its connection. This process will get the latest available transactions from your bank into your account.
Here's how to do it:
After updating, check your transactions again.
However, if you encounter an error code or error message, you can refer to this article about fixing bank upload errors in QuickBooks Online. It has specific steps for the message you see.
Additionally, you can get transactions from your bank or credit cards and manually import them into QBO.
If you need help with other tasks in QBO, browse this link to go to our general topics with articles.
Please know that you're always welcome to reply anytime if you still have questions or concerns with bank accounts. I'll be around for you. Take care and have a wonderful day ahead.