QuickBooks Error 6123 is caused by a damaged or corrupt company file. This error can also be caused by issues with network connection, firewall settings or antivirus blocking QuickBooks, or outdated QuickBooks software. This error can prevent users from opening or accessing the company file, and may cause data loss. Resolving this error typically involves restoring a backup of the company file, running the QuickBooks File Doctor tool, or repairing or reinstalling QuickBooks software.