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.
The WebDAV “make folder” method, MKCOL, requires the “Add Folders” permission. This is not normally granted to Members or Owners on the site.
In the Zope Management Interface, under the “Security” tab for the Plone root, check the “Owners” box for the “Add Folders” permission setting.