Download fails: External Storage App with SMB

Nextcloud version: 12.0
Ubuntu 16.04.2
nginx 1.10
PHP version 7.0.18
Is this the first time you’ve seen this error?: yes!

Can you reliably replicate it? (If so, please outline steps):
Nextcloud-Server: fs-cloud.f1.htw-berlin.de
=> with external storage app
=> with loaded SMB Folders per user
=> with login authentification over SAMBA-Server
SAMBA-Server: fs-daten.f1.htw-berlin.de

=> External, user-specific folders are shown in Nextcloud: YES!
=> cronjob does every 5min occ files:scan -all
=> Login into Nextcloud and downloading a 5GB big file from within the external folder: WORKS!

The issue you are facing:

The output of your Nextcloud log in Admin > Logging:
=> no new entry!

The output of your config.php file in /path/to/nextcloud:

<?php $CONFIG = array ( 'instanceid' => '***', 'passwordsalt' => '***', 'secret' => '***', 'trusted_domains' => array ( 0 => 'fs-cloud.f1.htw-berlin.de', ), 'datadirectory' => '/usr/share/nginx/nextcloud-data', 'overwrite.cli.url' => 'https://fs-cloud.f1.htw-berlin.de', 'dbtype' => 'mysql', 'version' => '12.0.0.29', 'dbname' => '***', 'dbhost' => '***', 'dbport' => '', 'dbtableprefix' => 'oc_', 'dbuser' => 'nextcloud', 'dbpassword' => '****', 'logtimezone' => 'Europe/Berlin', 'default_language' => 'de', 'installed' => true, 'memcache.local' => '\\OC\\Memcache\\Redis', 'memcache.locking' => '\\OC\\Memcache\\Redis', 'redis' => array ( 'host' => 'localhost', 'port' => 6379, ), 'mail_from_address' => '***', 'mail_smtpmode' => 'php', 'mail_domain' => 'htw-berlin.de', 'maintenance' => false, 'user_backends' => array ( 0 => array ( 'class' => 'OC_User_SMB', 'arguments' => array ( 0 => 'fs-daten.f1.htw-berlin.de', ), ), ), 'theme' => '', 'loglevel' => 2, 'updater.secret' => *** );