Very weird, I’m having the same issue after “moving” the plex startup registry from HKLU to HKLM. It apperead in msconfig.
I then “moved” it to Startup folder (first on all users, then on current user). It appeared in msconfig, but did’t start.
Then I remove all the changes I made and enabled the run at startup ticket in Plex tray icon, but didn’t work. I think I’ll need to reinstall Plex.
It went bad after I realized that I need to run Xteve with admin privileges for Plex to be able to build the IPTV Guide. After trying to make both processes to run at startup with admin privileges, neither one launched at startup again. I can’t understand why…