Why there is always someone that comes out with such silly comments? Well, back to the point. Thank you RomanZ for your response. Those errors are able to survive reboots, but following your suggestion I tried a simple app that triggers the rescan and it works. I don't know if my issue is widespread or not. Anyway, if I take a picture, it gets added to the gallery, so other apps do the update. I think that btsync too should request the update, maybe avoiding more than a request per hour or so. Meanwhile, I'm happy with this manual workaround. Thank you again for your support.