I reported this vnc error on :
Unfortunately nobody was interested in that topic . But as least I found a temporary solution myself.
When a user log in locally to the desktop GUI:
When the same user login to tiger vnc server remotely:
Plank won't work on the vnc user due to this error. I don't think this is a vnc issue, I googled online that this should be pam_systemd issue. I couldn't find any solution though. I really wish XDG_SESSION_TYPE can be modified on vnc server side. Hopefully someone could solve this issue in the next Ubuntu version. Cheers.