From FAQ:
What can I do if I don't want to use Dropbox.com?


(setq org-mobile-directory "/scpc:user@@remote.host:org/webdav/")

Having the '@@' caused the username to be 'user@' so I could not log
in, removing one of the @'s solved the problem.

Best wishes,

Enda


P.S.

Can TRAMP use WebDAV?