[Zope] Webdav and winword

He folks, I try to create a folder in Zope for some folks with winword for shared documents. I thought we could use webdav to access those files and edit them... But each time I open a file from within winword vi the webdav port, winword opens the file in read-only mode. If I create a new file and save that multiple times, everything works fine. Sometimes I even can open the file in read-write mode and than suddently I can only open them read-only. I checked in "WebDAV Lock Manager" but the file is not locked. Any Ideas or hints for debugging ? Regards, Volker Wend