The current command fails (git-annex 5.20141125, debian sid, DAV server: radicale 0.9-1):

    $ WEBDAV_USERNAME=user WEBDAV_PASSWORD=password git annex --debug initremote dav.example.com type=webdav url=https://dav.example.com encryption=none
    [2014-11-28 16:53:29 CET] read: git ["--git-dir=/home/user/test-annex/.git","--work-tree=/home/user/test-annex","show-ref","git-annex"]
    [2014-11-28 16:53:29 CET] read: git ["--git-dir=/home/user/test-annex/.git","--work-tree=/home/user/test-annex","show-ref","--hash","refs/heads/git-annex"]
    [2014-11-28 16:53:29 CET] read: git ["--git-dir=/home/user/test-annex/.git","--work-tree=/home/user/test-annex","log","refs/heads/git-annex..f60365a0d5333e1aefe80a3fe747e2cb3d5c8162","-n1","--pretty=%H"]
    [2014-11-28 16:53:29 CET] chat: git ["--git-dir=/home/user/test-annex/.git","--work-tree=/home/user/test-annex","cat-file","--batch"]
    initremote dav.example.com (testing WebDAV server...)

    git-annex: WebDAV test failed: StatusCodeException (Status {statusCode = 500, statusMessage = "Internal Server Error"}) [("Server","nginx/1.2.1"),("Date","Fri, 28 Nov 2014 15:53:37 GMT"),("Content-Type","text/plain"),("Content-Length","59"),("Connection","keep-alive"),("Keep-Alive","timeout=75"),("X-Response-Body-Start","A server error occurred. Please contact the administrator."),("X-Request-URL","MKCOL https://dav.example.com:443/tmp")] (CJ {expose = []}): user error
    failed
    git-annex: initremote: 1 failed

server log:

    ::ffff:192.168.1.6 - user [28/Nov/2014:16:53:17 +0100] "MKCOL / HTTP/1.1" 500 59 "-" "hDav-using application"
    ::ffff:192.168.1.6 - user [28/Nov/2014:16:53:18 +0100] "PROPFIND /tmp HTTP/1.1" 500 59 "-" "hDav-using application"
    ::ffff:192.168.1.6 - user [28/Nov/2014:16:53:21 +0100] "MKCOL /tmp HTTP/1.1" 500 59 "-" "hDav-using application"
    ::ffff:192.168.1.6 - user [28/Nov/2014:16:53:32 +0100] "MKCOL / HTTP/1.1" 500 59 "-" "hDav-using application"
    ::ffff:192.168.1.6 - user [28/Nov/2014:16:53:34 +0100] "PROPFIND /tmp HTTP/1.1" 500 59 "-" "hDav-using application"
    ::ffff:192.168.1.6 - user [28/Nov/2014:16:53:37 +0100] "MKCOL /tmp HTTP/1.1" 500 59 "-" "hDav-using application"
    ::ffff:192.168.1.6 - user [28/Nov/2014:16:58:31 +0100] "PROPFIND / HTTP/1.1" 207 1035 "-" "cadaver/0.23.3 neon/0.30.1"
    ::ffff:192.168.1.6 - user [28/Nov/2014:16:59:19 +0100] "MKCOL / HTTP/1.1" 500 59 "-" "hDav-using application"
    ::ffff:192.168.1.6 - user [28/Nov/2014:16:59:21 +0100] "PROPFIND /tmp HTTP/1.1" 500 59 "-" "hDav-using application"
    ::ffff:192.168.1.6 - user [28/Nov/2014:16:59:23 +0100] "MKCOL /tmp HTTP/1.1" 500 59 "-" "hDav-using application"


cadaver works:

    $ cadaver https://dav.example.com
    Authentication required for Radicale - Password Required on server `dav.example.com':
    Username: user
    Password: 
    dav:/> ls
    Listing collection `/': succeeded.
    Coll:   user                                  0  Jan  1  1970

server log:

    ::ffff:192.168.1.6 - - [28/Nov/2014:17:01:31 +0100] "OPTIONS / HTTP/1.1" 200 0 "-" "cadaver/0.23.3 neon/0.30.1"
    ::ffff:192.168.1.6 - - [28/Nov/2014:17:01:31 +0100] "PROPFIND / HTTP/1.1" 401 0 "-" "cadaver/0.23.3 neon/0.30.1"
    ::ffff:192.168.1.6 - user [28/Nov/2014:17:01:42 +0100] "PROPFIND / HTTP/1.1" 207 579 "-" "cadaver/0.23.3 neon/0.30.1"
    ::ffff:192.168.1.6 - user [28/Nov/2014:17:01:42 +0100] "PROPFIND / HTTP/1.1" 207 579 "-" "cadaver/0.23.3 neon/0.30.1"
    ::ffff:192.168.1.6 - user [28/Nov/2014:17:02:01 +0100] "PROPFIND / HTTP/1.1" 207 1035 "-" "cadaver/0.23.3 neon/0.30.1"