QuickBooks Error 6129 occurs when users attempt to create a new QuickBooks company file or open an existing one. This error is usually triggered by network issues, particularly when the network connection to the database server is disrupted or unstable. Additionally, discrepancies in network data files or outdated QuickBooks software can also cause this error. Users experiencing this issue may face difficulties in accessing their company files, leading to potential disruptions in managing their financial data efficiently.