(defun contact-server ()
  "Contact server."
  (let* ((username "damien")
         (password "the password")
         (authorization (format "Basic %s" (base64-encode-string (format "%s:%s" username password))))
         (url "https://damien@some-nextcloud.foo/personal")
         (url-request-method "OPTIONS")
         (url-request-data nil)
         (url-request-extra-headers `(("Authorization" . ,authorization)))
         (buffer (url-retrieve-synchronously url)))
    (with-current-buffer buffer
      (goto-char (point-min))
      (when (not (re-search-forward "^HTTP[^ ]* \\([0-9]+ .*\\)$"
				    (point-at-eol) t))
        (user-error "No valid HTTP response from URL %s" url))
      (let ((response (match-string 1)))
        (when (not (string-match "2[0-9][0-9].*" response))
	  (user-error "Error while checking for OPTIONS at URL %s: %s" url response)))
      (mail-narrow-to-head)
      (let ((davheader (mail-fetch-field "dav")))
        (when (not davheader)
	  (user-error "The URL %s does not accept DAV requests" url)))
      (message "Success contacting server"))))

(progn
  (message "Contacting server (1/2)")
  (contact-server)
  (message "Contacting server (2/2)")
  (contact-server))