QuickBooks error 6123 occurs whenever users try to restore, open, or upgrade a company file. The error occurs because firewall rules prevent access to company files that users have corrupted. The problem occurs when users try to access company files through multiple computers on a shared network. When this issue occurs, an error message will appear on your screen stating, “QuickBooks 6123 Error: We’re sorry, but QuickBooks can’t open your company file.

https://www.promoteproject.com/article/210607/4-effortless-solutions-to-fix-the-quickbooks-error-6123