I've deleted the objects folder, but I can't uninstall Android SDK Build-tools 21.1.2 (as said here, point 3) because the sdk manager doesn't have the uninstall option (in case that's relevant)
The thing is that I've just downloaded the project and this happened the first time. Also, the "device" is a just-created virtual device; I've installed nothing on it (literally)
However, I've tried your suggestion and... it worked!?