The most common causes of QuickBooks Error 400: HTTP Bad Request are improper request syntax or problems with server communication. It may interfere with syncing or online banking. It can be fixed by updating QuickBooks, checking internet settings, and confirming login information. Reinstalling the software or restarting the computer may potentially be effective ways to resolve this issue. Contact the helpful customer care line at 1.855.738.2784 if you require assistance with QuickBooks.