According to Intuit, Error Code 6123 indicates that QuickBooks didn’t receive a reply from the computer on which the QuickBooks company file is located. It occurs when QuickBooks is operating in multi-user mode.
The file path at which QuickBooks expects to find the company file is the location shown on the Product Information screen. This screen can be displayed by pressing F2 while in QuickBooks.
For more information on resolving Error Code 6123, consult this Intuit knowledge base article.