I took this one, i don't think there is a new one :
http://www.freenas.org/downloads/plugins/9/x64/syncthing-0.8.15-amd64.pbi
And after installing, when i wanted to start it from the plugin management webui of FreeNAS, the service wouldn't start.
After trying in the shell, i had the aforementionned error.
Well anyway, after creating syncer it works :) But i won't be using this program (as remplacement of btync) as i've just found out you can't pause transfers from the webui (i have a very slow connection and need to pause sync when i'm using the bandwidth, and pausing ability needs to be totally WAF)