All we can say in this Apple forum is that downgrading the iOS is not supported by Apple.
If you want to sync to a computer with iOS you need:
Syncing with iTunes on a Mac or PC requires:
- Mac: OS X v10.6.8 or later
- PC: Windows 7; Windows Vista; or Windows XP Home or Professional with Service Pack 3 or later
- iTunes 10.7 or later7 (free download from www.apple.com/ae/itunes/download)
Found this on another similar topic and it works!
as I found out from other threads in this forum and information gathered elsewhere downgrading iOS6 is not necessary. It is sufficient to download iTunes 10.6.3:
This iTunes Version is compatible with MacOS 10.5.8 and G4 processors and also able to sync with iOS6 devices. As it is not the latest version AppleUpdate does not install it automatically, so you have to download manually and install it from the diskimage.
Works and my sons iPod-MacMini combo is "saved".