503 (Service unavailable) with AJAX PROPFIND-request (worked fine with owncloud)

Nextcloud version: 12.0 Beta 3
Operating system and version: Debian 9
Apache or nginx version: nginx/1.10.1
PHP version: 7.0
Is this the first time you’ve seen this error?: yes

Can you reliably replicate it?: On my nextcloud instance, when doing a PROPFIND-request via javascript-ajax I get a 503 Service unavailable error.

The issue you are facing:
I’m using the following code to send a propfind-request through my php-proxy to my nextcloud instance:
$.ajax({ type: "PROPFIND", url: "/ilias/proxy.php", headers: { "X-Proxy-URL": ocBaseUrl + filePath, "Authorization": "Basic " + ocAuth }, success: function (data) { item.storageLastModified = new Date($(data).find("getlastmodified").text()); ... }, error: function (jqXHR, textStatus, errorThrown) { if (jqXHR.status == 404) { item.storageLastModified = "-"; ... } else { console.log("Error reading " + ocBaseUrl + filePath); } } });
I recently migrated from ownCloud to nextcloud and the requests worked fine when I was using owncloud.

The output of your Nextcloud log in Admin > Logging: No entries

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php $CONFIG = array ( 'instanceid' => 'xxx', 'passwordsalt' => 'xxx', 'secret' => 'xxx', 'trusted_domains' => array ( 0 => 'xxx', ), 'datadirectory' => '/var/www/xxx/nextcloud/data', 'overwrite.cli.url' => 'xxx', 'dbtype' => 'mysql', 'version' => '12.0.0.20', 'dbname' => 'owncloud', 'dbhost' => 'localhost', 'dbtableprefix' => 'oc_', 'dbuser' => 'xxx', 'dbpassword' => 'xxx', 'installed' => true, 'theme' => '', 'maintenance' => false, 'loglevel' => 0, 'mail_smtpmode' => 'php', 'trashbin_retention_obligation' => 'auto', 'updater.secret' => 'xxx', 'updater.release.channel' => 'beta', );

The output of your Apache/nginx/system log in /var/log/____: No entries in nginx error.log

I hope that someone can help me with this.

Greetings, Sabbertran