Plex - service keeps stopping

Having trouble with Plex Media Server service - stops just after start.

- ReadyNAS Ultra 2
- Radiator 4.2.26
- Current Plex Media Server installed: 9.9.11

Using Plex version 9.9.11 for while now, without issues. Tried installing latest add-ons - 9.9.12 and 9.9.13
Each time install add-on, the service does not start.
From Frontview, I go to 'Installed' add-ons and try enabled Plex server ...

Tick the box and apply, the dot turns green briefly and back to grey.
Hover mouse over and says running.
Try to connect to Plex over LAN and WAN - no connection ...

When the dot turns grey, the box is still ticked. If untick and apply then get the following message:

"PlexMediaServer service was already stopped or the service failed to stop"

The Plex Media Server.log shows the test below:

Aug 02, 2014 15:11:05 [0xf536f6c0] INFO - Plex Media Server v0.9.9.13.525-197d5ed - debian PC i686
Aug 02, 2014 15:11:05 [0xf536f6c0] INFO - SQLITE3:14, 283, recovered 217 frames from WAL file /c/.plex/Library/Application Support/Plex Media Server/Plug-in Support/Databases/com.plexapp.plugins.library.db-wal
Aug 02, 2014 15:11:05 [0xf536f6c0] ERROR - Error: Unable to set up server: bind: Address already in use

On have tried Plex forums, but many say this is due to another service using same port - and not a Plex issue.
I find this odd, as I have been using Plex for years now and always upgrade to latest version (plexpass versions and public version).
If using Plex Server 9.9.11 and works fine, then upgrade to 9.9.12/13 and have issue.
When downgrade to 9.9.11, all is ok again.

Has anyone on here experience similar issue?
How to I check what ports are in use on the ReadyNAS?
If anyone needs to see some logs, just let me know....

Thanks,

Brez