webDAV error with nginx

Hello everyone,
after a Server reinstall on the new Ubuntu LTS i’ve got a few Problems with the WebDAV acces

The Webinterface works fine but the Desktop Client produces Errors and do nothing else.

The Server runs on the actuall Ubuntu LTS release with PHP7, Nginx an MySQL

The WebDAV produses the following error

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(60):
Sabre\DAV\Server->exec()\n#6 /var/www/owncloud/remote.php(165):
require_once(’/var/www/ownclo…’)\n#7
{main}",“File”:"/var/www/owncloud/apps/dav/lib/Connector/Sabre/File.php",“Line”:307,“User”:“Tobias”}

ideas?
thank

What client are you using? OSX?

The Nextcloud version would be interesting as well. If you want to check your client, you can try to connect via webdav to demo.nextcloud.com.