Hello everyone!

I require a bit of assistance!

My son wants to use OpenProject on a Raspberry Pi 4. As he has 8 GB of RAM, he requires a 64-bit operating system. 
Since the original operating system of the Rasberry Pi is only available as a 32-bit, he decided on installing a new one - the 64-bit verion of Ubuntu 20.4. 
He used the tutorial on https://www.madewhatnow.com/OpenProjectRaspberryPi/ to set up a working installation of OpenProject.

To assist him with any occurring problems, I set up a VM with his used operating system and used the tutorial to successfully install OpenProject.

Soon my son wished to host additional webapplications other than OpenProject, like a Wiki. But since his Raspberry System was already used/worked on productively, 
I tried reconfiguring the installation on my VM first.
Since VMs create snapshots frequently it would be easy to go back to previous working versions.

After some research I found instructions in this forum to add the Sub-URL as a value of "rails_relative_url_root" in the configuration-file ("config/configuration.yml").
After I did that I deleted the directory "public/assets" and recompiled it ("./bin/rake assets:precompile") 
and adjusted the configuration of the apacheserver to access OpenProject via the sub-URL "/openproject" by using the product passenger.

But if I try to access "http://myhost.de/openproject/" in a browser I don't see the homepage I should see, but the text "Not Found /" after waiting a few seconds.

I've already tried using different versions of OpenProject ("stable/11", "release/11.2"), just to try if those work, but they also don't work.
I've also tried using different versions of Ruby and "npm" but had no success.

As a last resort I also tried installing the precompiled package for Ubuntu 20.4 "x86_x64" from zero.
With that package installed could access OpenProject via the root-URL ("http://myhost.de/"). 
After reconfiguring using the installation-software to use the sub-url "/openproject", I could also access OpenProject via that URL ("http://myhost.de/openproject").
Unfortunately there is no precompiled package available for ARM CPU and Ubuntu 20.4, so that solution would not work for my son.

I have already activated debug-messages in the passenger of my apache webserver, but those don't help me at all.

I am grateful for every piece of advice you give me!

Greetings
Reiner