Nextcloud and SWAG "unstable"

In order to be able to connect to my Nextcloud docker via HTTPS (some apps on my phone complained about the self-signed certificate), I switched over to SWAG+Nextcloud and used this tutorial:

https://medium.com/@chrismorris_82249/setup-a-reverse-proxy-nextcloud-server-on-unraid-using-letsencrypt-831905d94f7d

So far though, the whole setup has been a bit unstable.

This is my Nextcloud config.php:

<?php
$CONFIG = array (
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'datadirectory' => '/data',
  'instanceid' => 'octfw6m2dcll',
  'passwordsalt' => '*',
  'secret' => '+*',
  'trusted_domains' => 
  array (
    0 => '192.168.178.73',
    1 => 'pillendrehertestnextcloud.duckdns.org',
  ),
  'trusted_proxies' => ['swag'],
  'overwrite.cli.url' => 'https://pillendrehertestnextcloud.duckdns.org',
  'overwritehost' => 'pillendrehertestnextcloud.duckdns.org',
  'overwriteprotocol' => 'https',
  'dbtype' => 'mysql',
  'version' => '20.0.2.2',
  'dbname' => 'nextcloud',
  'dbhost' => '192.168.178.73:3306',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'nextcloud',
  'dbpassword' => '*',
  'installed' => true,
  'app_install_overwrite' => 
  array (
    0 => 'documents',
  ),
);

And this is what SWAG's nextcloud.subdomain.conf looks like:

## Version 2020/12/09
# make sure that your dns has a cname set for nextcloud
# assuming this container is called "swag", edit your nextcloud container's config
# located at /config/www/nextcloud/config/config.php and add the following lines before the ");":
#  'trusted_proxies' => ['swag'],
#  'overwrite.cli.url' => 'https://nextcloud.your-domain.com/',
#  'overwritehost' => 'nextcloud.your-domain.com',
#  'overwriteprotocol' => 'https',
#
# Also don't forget to add your domain name to the trusted domains array. It should look somewhat like this:
#  array (
#    0 => '192.168.0.1:444', # This line may look different on your setup, don't modify it.
#    1 => 'nextcloud.your-domain.com',
#  ),

server {
    listen 443 ssl;
    listen [::]:443 ssl;

    server_name pillendrehertestnextcloud.duckdns.org;

    include /config/nginx/ssl.conf;

    client_max_body_size 0;

    location / {
        include /config/nginx/proxy.conf;
        resolver 127.0.0.11 valid=30s;
        set $upstream_app nextcloud;
        set $upstream_port 443;
        set $upstream_proto https;
        proxy_pass $upstream_proto://$upstream_app:$upstream_port;

        proxy_max_temp_file_size 2048m;
    }
}

I've been experiencing a couple of issues. But the most frustrating one is this: I am able to access my nextcloud instance via "pillendrehertestnextcloud.duckdns.org", but if I open the Nextcloud-docker's Webui via the Unraid webif, the redirect to pillendrehertestnextcloud.duckdns.org only happens about half of the time. Last night on my Desktop it worked. This morning my laptop gets sent to 192.168.178.73:444 and Firefox complains about the self-signed certificate. I even checkd with Edge just to make sure that it's not a cache/whatever problem with Firefox, but it didn't work there either. And just to make sure, I switched on my Desktop PC and tried the Webui there: I get the redirect to pillendrehertestnextcloud.duckdns.org with Firefox (both in my standard profile and via private mode), but not with Edge.

So what gives? I thought the 'overwrite.cli.url' part of the Nextcloud config was supposed to redirect everything to my duckdns.org url?