Problems with accessing Nextcloud via internet

After my old installation suddenly didn’t work anymore from one to the other day I chose to set up a completely new instance of Nextcloud.
But now I’m totally stuck as I’m facing the following problem:
Accessing my Nextcloud installation from the local network works flawlessly but when I try to access it from the internet (or the mobile app) I get a login page without the background image and a lot of javascript/jquery errors in the Firefox console.
I checked with my old installation and did everything as before (checked all the configuration files) and according to a lot of installation manuals from the web but after a lot of research and wasted time I’m left completely clueless. There is something I’m missing or someting I have to activate

What I tried so far/my configuration, all of the combinations give me the same result:
operating systems: Ubuntu Server Raspberry/Ubuntu Mate Raspberry/Raspbian
Nextcloud versions: 9/10/11
Server: Apache
PHP versions: PHP5/PHP7
Database: MariaDB
SSL certificate via Let’sEncrypt
Port Forwarding in router is activated

I’m really glad for any useful hints or solutions and will try to provide as much information as possible to get my Nextcloud accessible from the internet/mobile app again.

Thanks a lot for your help, Joni

Can you output the contents of your config.php file (without password salt, ID, passwords, etc)? It’ll look something like this blanked out:

Additionally if you use Chrome, Firefox or another browser with page inspection, right click on the broken page, click inspect element and look out for the errors:

This will potentially give an indication of the path your install is trying to load scripts from.

It sounds to me like it’s been installed to be accessed via internal IP and the overwrite.url is the local IP rather than a FQDN.

This is my config.php

And the errors from Firefox page inspector.

And from Vivaldi.

Hmm, could you message me your URL (I’ll message you to reply, if you choose to to overcome the restrictions on new users)?

Is the server behind a reverse proxy, or do you have any special firewalling in place?