There is an untested hypothesis that RequireSignOrSeal may cause windows XP to fail to authenticate to a non ssl webdav server (please update if you prove either way) - AdrianKitto
At this point in time I do not know anything about WebDav I am just making this point as Yem in #NZADSL was having an issue with getting WebDav to work under MicrosoftWindows XP and this was a suggested fix. Awaiting feedback.
2 pages link to WebDav:
lib/main.php:944: Notice: PageInfo: Cannot find action page
http://www.webdav.org/