Itunes - Convert higher bit rate songs to 128kbps AAC?

I have an iPhone 4S, i've had it a while with no problems, then when I synced my phone today and tried to 'autofill' the library or whatever it came up with this pop up saying iTunes can't add any songs… To allow songs to be copied to this iPhone, turn on "Convert higher bit rate songs to 128kbps AAC" for this iPhone. So I did, and then tried to autofill and sync again and this time it just said "iTunes can't add any songs to the iPhone because no songs in your library can be copied"… What? Well they've been fine to be copied until now!
What's going on, and how do I fix it?

It's probably a bug. Try to upgrade iTunes or else try again later. Sometimes it just acts up. It's really buggy in Windows IMO.