Hello,
I apologize for the delay in my reply, but after reading your Qnap FAQ, I found a lot of bad configurations in my installation. So I made the decision to reinstall all from scratch.
I completely removed the package and the directory:
/share/CACHEDEV1_DATA/.qpkg/PlexMediaServer/
then I made a new fresh installation from latest stable release and I solved all my problem.
Now my QNAP correctly start Plex at every scheduled reboot.
Thank you very much for the support.
Best regards.
GreatGonzo