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Problem solved. An update / upgrade process solved the issue. No idea why but it did.
I have the same problem. Everything worked well during installation and after that for some time. I could login and everything.

However, I set the server aside for a while and didn't use it. When I tried to login again, port 8000 was not available. The webserver is still running, as well as the applications, it is just not possible to access the Ajenti panel anymore.

Tried "sudo netstat -tlnp | grep 8000" and it did... nothing.

Access is tried with https.

Not sure what happened.