Can't sync even though I've got enough space

I've got enough space on my iPhone but iTunes just refuses to sync:

This has been happening for a couple of weeks.

It usually gets normal after I restart iTunes, but today it just won't work. Tried relaunching iTunes / iPhone, but.

Does anyone know how to fix this?

I'm using iPhone 5 (iOS 8.1.3), iTunes 12.1.2.27, MacBook Pro OS X Yosemite