Hi DaniDawg,
Based on history, this happens when PayPal is connected to QuickBooks via online banking and via PayPal app. you only need to choose one so you won't get Error 400. If this is not the case with you, you can do the browser troubleshooting steps.
To start, you'll want to sign in to QuickBooks using a private browser. This step disables its extensions and stops the cache from saving your browsing history.
- Google Chrome: Ctrl + Shift + N
- MS Edge and Firefox: Ctrl + Shift + P
- Safari: Command + Shift + N
If you don't see the error anymore, you can go back to your regular browser and clear its cache. If not, please try other browsers to see if we get the same thing.
Let me know if you need further assistance.