On Mac OS X Server 10.3 to 10.3.9, WebDAV allows you to create, copy, and delete files on WebDAV volumes, but not rename them. If you try to rename a file or folder on a WebDAV volume that is on the default "wild-card" virtual host, you will get an Error -43 message. This issue does not affect volumes on named virtual hosts.
As a client-centric workaround, you can copy the file to a non-WebDAV volume, change the name of the copied file, copy it back to the WebDAV volume, and then delete the original file.