besides being synced with another library...
the only thing i can think of is to erase itunes, erase mobile device helper, erase the lockdown folder. and reinstall. do not erase your tunes library.
take it to the genius.
the only thing that matches this problem the OP ( edit : that i heard of ) is having is Snow Leopard and iTunes 11.4 , it had a bug and it wouldn't work on 32 bit machines, but the OP saying he is running 12.01 disproves this.
so i am stuck
i don't know how to help you , call applecare
tell them i want an exemption , i recently installed IOS 8 and they won't charge you $29 for the call
if you go to support.apple.com
and tell them you want to contact apple support.
there is an option there to send them iOS diagnostics.
they will send you a text message so you can submit an extended test
that might show something relevant.
they will give you a case # to give to the applecare person, using GSX
they can also access your screen on your mac ( with permission of course ) so help you troubleshoot.
this is all potentially FREE because of your iOS 8 exemption, heh
another weird thing i found out recently is that if you activate a USED iPhone off of eBay
you get 90 days of free technical support., but of course you don't get service/repairs