I had a trouble adding my existing multimedia content to plex library. It turns out that the media files were not visible to plex due to mount point of my HDD (/medina//HDD).
After mounting the HDD at /mnt/HDD the media files are visible and the server is working just fine.
While debugging I changed the default groups of the plex user to see if it is problem with the access permissions. Now plex user is part of root, sudo and many other groups where it is not meant to be.
I want to remove all the extra groups that plex user was added. So please tell me in which groups plex user belongs to on default installation.