I think you have an old version of the oacs-dav package.
The problem is a bug in Web Folders. If you do not authenticate when the folder is opened, it does not ask for your password when you try a write operation such as copying a file into the web folder.
So, i disabled anonymous read on the OpenACS WebDAV folders. This means authentication is required to get the directory listing which works around this problem. I added a prameter to turn this off, but it is enabled by default so Web Folders should work.
You should try the latest version on the oacs-5-1 branch in CVS.
