Files_external not working

I have installed nextcloud from git and I am trying to enable files_external app. Unfortunately I get this:

{“reqId”:“IhcptQS9goAGiilCay5i”,“level”:3,“time”:“2018-08-16T06:38:53+00:00”,“remoteAddr”:“172.25.0.2”,“user”:“root”,“app”:“PHP”,“method”:“GET”,“url”:"/",“message”:“Interface ‘OCA\Files_External\Lib\Config\IBackendProvider’ not found at /var/www/html/apps/files_external/lib/AppInfo/Application.php#68”,“userAgent”:“Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:50.0) Gecko/20100101 Firefox/50.0”,“version”:“14.0.0.15”}

this is weird as the file is there:
root@43276ea34897:/var/www/html# md5sum apps/files_external/lib/Lib/Config/IBackendProvider.php
ad9d2c8f6f1d8d9d6c6232e64f4cd02d apps/files_external/lib/Lib/Config/IBackendProvider.php

Any idea why the interface cannot be found?

UPDATE:
If I add this line:
require(DIR . ‘/…/Lib/Config/IBackendProvider.php’);
nextcloud loads. The weird thing is that no similar line is required for IAuthMechanismProvider. Still, this is more of a workaround than a solution.