Are you using QuickBooks software and error 1603 is coming up? The QuickBooks error 1603 can appear if you are connecting with the wrong credentials or if you are sharing a network with another person who is already connected to the QuickBooks servers. This error also usually occurs when you try to access the program after an update or restore or when you have multiple instances of the program running on your computer at one time and they're not working with each other or is likely caused by a corrupted file due to hardware or software failure.