When accessing a folder via WebDAV on a SharePoint site behind Threat Management Gateway the end user will occasionally prompted for windows credentials. The site being accessed uses forms based authentication using persistent cookies while the connection TMG utilizes for SharePoint is NTLM. Under normal working conditions the user is not prompted for any credentials when using WebDAV after first logging in.
The issue can be temporarily resolved by restarting the windows service called 'Web Client' but eventually the problem will come back. We've been able to recreate the issue by accessing a resource we don't have permission to read. Any following navigation requests will prompt the user for credentials.
The clients accessing the web folder are running Windows 7. The TMG server is running Windows Server 2008 R2 with SP 1. The SharePoint site is running Windows server 2003 and MOSS, though the issue still exists on our SharePoint 2010 Enterprise box that is running Windows 2008 R2 with SP1.