This is a mirror of the (excellent) documentation at http://plone.org/documentation/error/unable-to-create-a-folder-through-webdav/, but just a short editorial to also highlight that this is still very applicable to Plone 4.x. Effectively, the symptom is that users will report seeing a “401 Unauthorized” error when trying to create folders within areas they have access to as an Owner. Without further ado, the documentation:
Using a WebDAV client on a Plone site, site users are unable to create newfolders directories where they have Owner role.
Problem:
The WebDAV “make folder” method, MKCOL, requires the “Add Folders” permission. This is not normally granted to Members or Owners on the site.
Workaround:
In the Zope Management Interface, under the “Security” tab for the Plone root, check the “Owners” box for the “Add Folders” permission setting.