not writable

<VirtualHost *>
        ServerAdmin webmaster@localhost
        ServerName fritz.box
        ServerName webdav.fritz.box
        DocumentRoot /home/martin/webdav
        Alias /martin/ "/home/martin/webdav/"
        <Directory "/home/martin/webdav">
                Options Indexes MultiViews
                AllowOverride None
                Order allow,deny
                allow from all
        </Directory>
        <Location /martin>
              DAV On
              AuthType Digest
              AuthName "webdav-fritz.box"
              AuthUserFile /home/martin/webdav-dig-pw
              Require valid-user
        </Location>

        ErrorLog /var/log/apache2/error.log
</VirtualHost>