I'm at work right now so I can't try this, but I found this on an Apple Support Forum:
Gentlemen I've solved my problem (with the help of an other thread) !!
1. Download and install iExplorer (Google it)
2. Plug in iPhone first, then open iExplorer
3. Delete the iPhone\Media\iTunes_Control folder (you can copy it to your laptop, just in case)
4. Reboot the iPhone
5. Turn on iTunes Match in settings.
6. Start Music app
They also really reccomended reinstalling iOS 6 fresh without a backup. They say that iTunes match is much faster even with large libraries.
https://discussions.apple.com/thread/4311721?start=15&tstart=0
Macaddict16