The latest upgrade from Synology to DSM 6.1 will disable the Plex server.
This is how to resolve it.
My Synology DS213+, like many others, is no longer supported by Plex from PMS versions 0.9.17, so I have to stick with the older PMS version.
It turns out that the most recent Plex Media Server package for PPC (0.9.16.3.1840-cece46d) available in the Synology Download Center cannot be re-installed in DSM 6.1
Luckily there's a slightly newer build which works: 0.9.16.6.1993-5089475 and just needs to be installed manually after the DSM upgrade.
You can find a share to this file from Stikaree at the beginning of this thread from Plex forum.