Hi @tschabet ,
I edited your message to hide IP address and port.

I should maybe also hide your ticket system URL as you seems can have a potential security issue: directory listing.
Directory listing is good when you have a folder with no sensible data but having for folders where you have a web application installed can be dangerous.

First of all I suggest to disable directory listening and be sure about your Apache or / and server configuration.

I run UVdesk on Ubuntu 20.XX without issues so maybe something should be wrong on your install or server configuration.

I'm not expert in errors but maybe is something with your server / Apache configuration.
You can try to search on Google the error message you are getting: "AH01626: authorization result of Require all granted: granted"

Let us know once you have resolved. I hope this informations can help you.