Hi Ismael, thanks for your both hints.
The two clues I have according to your error are - Is .config/systemd/user created? (Maybe it has to be created manually)
Yes, I've dropped there the timer and service file.
- Does the second user have dbus started correctly? (for instance, does "systemctl --user status" work as intended for that user?) Maybe the second user has a non-working dbus service, or maybe it’s just an environment variable problem (if I remember correctly, XDG_RUNTIME_DIR is the variable that should be set correctly to something like /run/user/<userid>)
systemctl --user status has the same error. How do I start dbus for the second user? systemctl enable --user dbus fails due the same error, and system's dbus is running. Thanks Bjoern