i had this issue on linux w/ file mode and i believe i fixed it with stopping the btsync process and running:
./btsync --config /btapp/config/btsync.conf --identity Vostro
it should create an identity file, and then you can start btsync your usual way... at least this appears to have resolved it for me. seems like a bug with installing and only running btsync in file mode. if you run it in webui mode it seems to create this identity file initially.
it's frustrating that you can't configure btsync to use a configuration file AND webui mode together.