Explanation of fatal webdav error

Hey, I have some strange errors in my owncloud log.

Exception: {"Message":"HTTP\/1.1 503 Could not open file","Exception":"Sabre\DAV\Exception\ServiceUnavailable","Code":0,"Trace":"#0 \/var\/www\/owncloud\/3rdparty\/sabre\/dav\/lib\/DAV\/CorePlugin.php(83): OCA\DAV\Connector\Sabre\File->get()\n#1 [internal function]: Sabre\DAV\CorePlugin->httpGet(Object(Sabre\HTTP\Request), Object(Sabre\HTTP\Response))\n#2 \/var\/www\/owncloud\/3rdparty\/sabre\/event\/lib\/EventEmitterTrait.php(105): call_user_func_array(Array, Array)\n#3 \/var\/www\/owncloud\/3rdparty\/sabre\/dav\/lib\/DAV\/Server.php(459): Sabre\Event\EventEmitter->emit('method:GET', Array)\n#4 \/var\/www\/owncloud\/3rdparty\/sabre\/dav\/lib\/DAV\/Server.php(248): Sabre\DAV\Server->invokeMethod(Object(Sabre\HTTP\Request), Object(Sabre\HTTP\Response))\n#5 \/var\/www\/owncloud\/apps\/dav\/appinfo\/v1\/webdav.php(56): Sabre\DAV\Server->exec()\n#6 \/var\/www\/owncloud\/remote.php(138): require_once('\/var\/www\/ownclo...')\n#7 {main}","File":"\/var\/www\/owncloud\/apps\/dav\/lib\/connector\/sabre\/file.php","Line":306,"User":"owncloud_username"}

Another strange that owncloud_username has a problem with desktop synchronization client.
Only some of files which are store on server are synchronized with desktop. For instance from 12 folders synchronized only 4.

Does this problem connected with webdav error?