First of all, sorry for my english
I have installed first Plex from Synology application portal.
Then I saw an update was available. I downloaded the package and manually installed it.
Then, I tried to add a directory in the library but without success.
I decided to uninstall plex (from the application portal button).
From that time, when I now try to install it again from the regular way, I get a message informing that a service is already running and using the port 32400.
Thanks indeed. Very useful hints. After 2 days of searching for real help your post is the solution for strange broken links in my plex installation . Great !