Hi There,
So I installed the add-on via the plugin logged in as root.
I’m not sure how to find what user Syncthing is running under. I would have thought by default it would of allowed itself access to the FreeNAS file system! 
When creating a new Sync on the config page, I am told mkdir Permission denied on /mnt/Volume1 I presume it is doing an mkdir as it cannot verify a directory called Volume1 because it does have rights thus by default trying to create one which of course it shouldn’t be doing!
I’ve gone back a directory to /mnt which gives a different response “/mnt permission denied” without the mkdir.