I have had this problem repeatedly for the past 3 months. as a consequence of repeatedly resynching my Spotify Library I have exceeded my monthly ISP downolad allowance. I am therefore paying for the failure of Spotify to fix this bug.
I am off to Google and and suggest every other frustrated Spotify User does the same.
Mid 2017 already and I haven't found an explanation for this yet. The button keeps switching to off every month or so. And the annoying part is that if I start downloading my songs again, the app saves them in the internal storage instead of the external SD (Android device) so I have to re-install the app every time this happens in order for the app to recognize the external SD. (Sorry for bad english)