Have you tried removing it first, and then reinstalling from the command prompt? You will need a terminal emulator Cryptoterm is what I would recommend if you are on a windows computer. If you are on a mac then just open a terminal window and ssh directly to the ip address on the vortexbox. For Cryptoterm use the hostname which should be vortexbox if you have not changed it. Type in the commands "yum -y clean all;yum -y update all" without the quotation marks at the terminal prompt. Reboot the vortexbox after this is finished. Then you can try reinstalling plex from the terminal window by typing in "yum -y reinstall plexmediaserver" After it reinstalls issue the command "
service plexmediaserver start" in the terminal window. If this does not fix it, run a "yum -y erase plexmediaserver" and then try installing it again with a "yum -y install plexmediaserver"