You're not using the right version of the web app.
is giving me the same error
The answer is the same. You're not using the matching version of the web app.
I have upgraded to version 5.3. The operating system is Ubuntu 22.04.1. And now I get the message 404 - not found.
You're probably doing something wrong, but without any details it's impossible to tell what exactly.
I have restored version 5.2 and the interface is available. I upgraded again to 5.3 and again no interface.
And again zero useful information. My guess is that you're not just using the standard setup. Do you have a custom or a proxy or something? If you want help, you have to provide as much details as possible.
If I want to add a new user I get this message: