Error 6123 0 popping up while running QuickBooks? Our team can fix it for you. We are a team of certified QuickBooks pro advisors who are well-versed with the reasons that stem up QuickBooks error 6123 0 and the steps that need to be taken to fix the problem. It appears with a notification message saying “connection to the QuickBooks company file has been lost” while trying to upgrade the QuickBooks to the latest version. The reasons could be a damaged QuickBooks program file, misconfigured firewall settings, damages Windows OS, or maybe multiple versions of QuickBooks database services are running.