DUDE! I found the solution to this. I admit, I only read about half of this thread, because you guys were leading me in the wrong direction.
Sorry if I am resurrecting a year-old thread, but it needs to be done.
Here's the problem: both Transmission and some other plugin you have (in my case it was Plex) have the same MAC address on their IP. If you try to set Transmission as a static IP address in the Jails (i.e. turn of DHCP), then you can fool yourself into thinking that you have fixed it. I forced Transmission and Plex to have separate, distinct IP addresses. But Transmission STILL WON'T START.
So I originally had them both set DHCP, so I put it back that way, and f*ck if they didn't both have the same IP address again... I went to my server (router in your cases) and looked at the MAC address reservations I had previously put in there. Plex was 02ffb000050b and Transmission was 02ffb000060b. I went back to the Jails and looked and lo and behold they were both set to the same MAC. Once I corrected that and restarted both Jails, the Plugins were fixed, and auto-started instantly.
Also, the reported bug of SLOW LOADING on the installed Plugins tab... this is the cause of that too. Fix your MAC addresses, and a new bug should be introduced that each new plugin should get a unique MAC address, not the same one as appears to be happening in FreeNAS 9.10.