I collected all open webdav issues here.

The cause of all these issues are the webdav lib we are using is buggy, it's not working with some webdav server, we probably should consider use other libs, see eloy's comment: http://tracker.moodle.org/browse/MDL-22663?focusedCommentId=124685&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-124685