Th0mC It must be due to your set up. I see when you go to your main domain, it redirects to /forum. Maybe the manifest and sw.js needs to be moved to your main public root. Not in the /forum directory. And also revert the above changes. Put the start url and scope back to just “/.”
Another possibility is, I remember when troubleshooting this extension in the past, it takes a lot to clear all cached data. Just clearing the main browser cache might not be enough. I had to go into the Chrome dev console and actually delete things from the application tab. That’s if you are using Chrome. Long story short there are many things which could be causing this.
If you continue to not have success you may need to find a developer who is well versed in PWA. This extension works with a basic set up where the forum is served from the public root.