Syncthing by default has an auto update function so it can update the binary by itself, for instance, your addon has syncthing 0.11.9, but now theres available 0.11.11, so at start it checks for updates and then apply the new binary, in windows works fine.
On the nas doesn't appear to work.
I updated the binary myself with elfinder (or bash, don't remember), and it works, I now have syncthing 0.11.11 working.
But I assume that if there's another update it will fail again.
As I said, I think the reason is the user, because in Sickrage, Couchpotato updates work fine, I think beacuse they run under the user root, while syncthing runs as nobody.
Anyway, as I said, not really a bug nor a dealbreaker but just wanted to let you know.