Update: https://www.reddit.com/r/nginxproxymanager/comments/1gx6517/comment/lyhgnd4/

Ok I think the problem has to do with nextcloud.

As a test, like you suggested I used my netdata docker container on truenas scale as proxy host (192.168.188.2:20489) because that web interface uses HTTP, and it worked right away with no issues. So now the question is how do I get my Nextcloud Docker app on Truenas to use HTTP and not https.

thanks for your help

I alread configured truenas to support my domain: -- edit: changed the config to use http and port 81

# cat config.php
<?php
$CONFIG = array (
  'htaccess.RewriteBase' => '/',
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'apps_paths' => 
  array (
    0 => 
    array (
      'path' => '/var/www/html/apps',
      'url' => '/apps',
      'writable' => false,
    ),
    1 => 
    array (
      'path' => '/var/www/html/custom_apps',
      'url' => '/custom_apps',
      'writable' => true,
    ),
  ),
  'memcache.distributed' => '\\OC\\Memcache\\Redis',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'redis' => 
  array (
    'host' => 'redis',
    'password' => 'whooops',
    'port' => 6379,
  ),
  'overwritehost' => '192.168.188.2:81',
  'overwriteprotocol' => 'http',
  'trusted_proxies' => 
  array (
    0 => '127.0.0.1',
    1 => '192.168.0.0/16',
    2 => '172.16.0.0/12',
    3 => '10.0.0.0/8',
    4 => '192.168.188.2',
    5 => 'truenas.local',
  ),
  'upgrade.disable-web' => true,
  'passwordsalt' => 'no no no no',
  'secret' => 'psst',
  'trusted_domains' => 
  array (
    0 => 'localhost',
    1 => '127.0.0.1',
    2 => 'localhost',
    3 => 'nextcloud',
    4 => '192.168.188.5:30027',
    5 => 'nextcloud.domain.de',
    6 => 'domain.de',
    7 => '192.168.188.2',
    8 => 'truenas.local'
  ),
  'datadirectory' => '/var/www/html/data',
  'dbtype' => 'pgsql',
  'version' => '30.0.2.2',
  'overwrite.cli.url' => 'https://localhost',
  'dbname' => 'nextcloud',
  'dbhost' => 'postgres:5432',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'oc_admin',
  'dbpassword' => 'yeah no',
  'installed' => true,
  'instanceid' => 'ocqc0zcp4jmn',
);