According to Intuit, Error Code 6123,0 “Connection to the QuickBooks company file has been lost” occurs in multi-user environments caused by one of the following:
- the network connection between the computer on which the error occurred and the server hosting the company file was disrupted
- firewall software is blocking a port necessary to communicate with the server hosting the company file
- multiple instances of the QuickBooks database service are running
- QuickBooks data or program files are damaged
- the computer name on the computer hosting the QuickBooks company file was changed while hosting was turned on
- the McAfee Privacy Service is installed with the Block Web Bugs filter option selected
The last 2 possible causes are the easiest to troubleshoot. The Block Web Bugs filter in McAfee’s software is incompatible with QuickBooks and needs to be turned off. If the computer name on the computer hosting the company file was changed, close all applications, reboot that computer, and then first stop and re-start multi-user mode. Specific steps for these possible causes are discussed in this Intuit knowledge base article.
For specific troubleshooting steps to resolve the other possible causes of this error, consult this Intuit knowledge base article.