I have experienced this problem trying to start lxde using the TightVNC vncserver from systemd.
The process with the displayed PID is always lxsession.
I resolved it by adding PAMName=login to my unit file.
This suggestion came from https://bugs.archlinux.org/task/42425
Note: I did not have to specify "-fg" to vncserver, as also suggested in this bug (this option is anyway not supported by TightVNC).
I have experienced this problem trying to start lxde using the TightVNC vncserver from systemd.
The process with the displayed PID is always lxsession.
I resolved it by adding PAMName=login to my unit file.
This suggestion came from https:/ /bugs.archlinux .org/task/ 42425
Note: I did not have to specify "-fg" to vncserver, as also suggested in this bug (this option is anyway not supported by TightVNC).