Intuit has a free, downloadable tool to help diagnose and troubleshoot connectivity and multi-user issues. The QuickBooks Connection Diagnostic Tool can help solve certain 6000 errors and H202 and H505 errors.
Their tool is compatible with QuickBooks 2008, 2009, and 2010 versions (for Enterprise Edition users, versions 8, 9, and 10.) It works under Windows 7, Vista, and XP.
Learn about and download the tool here.