First:

Distributor ID: Ubuntu
Description:    Ubuntu 16.04.3 LTS
Release:    16.04
Codename:   xenial
Linux <hostname> 4.4.0-112-generic #135-Ubuntu SMP Fri Jan 19 11:48:36 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

I'm trying to start an instance of an application as a «user service» systemctl --user start <service>. But when I'm issuing this command (or any other systemctl --user-commands) I'll get an error saying

Failed to connect to bus: No such file or directory

It doesn't matter if I'm issuing this command as the root user or a normal user, but what I eventually want's is to run this as a normal user.

I have searched online and tried everything I can come across. One problem that many people with similar problems have mentioned is that $XDG_RUNTIME_DIR and $DBUS_SESSION_BUS_ADDRESS is not set for the environment; and Yes, it isn't in my case either, but the solution on setting them haven't helped me.

I have tried to set them after login and in .profile (also tried to reboot), but nothing helps.

Some mentioned that logging in to the user from the root-user (su - <username>) could also cause these problem, but the solutions here was to simply log in directly through SSH with the normal-user. But unfortunately it doesn't work either in my case.

Anyone with a suggestion on how I can get this working?

Yes. There is a reason why I want/need this for a particular user (or several), without giving it/them full root access.