Problem with synchronization (not all files synced)

Some of my files (on the server) are not synchronized with desktop client.

sudo -u www-data /var/www/owncloud/occ files:scan --all

doesn't help.

Operation cancelled. --->>> Log from client.

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"} --->>>log from server

Any idea?

Duplicate of https://central.owncloud.org/t/explanation-of-fatal-webdav-error/7739

Please avoid creating multiple threads about the same issue.