iTunes cannot sync information

Hi All

I keep getting the "iTunes cannot sync information with the iPhone “iPhone Name” because syncing has been disabled on this computer. Do you want to enable syncing?" error for 2 iPhones and an iPad. This started happening 2 weeks ago. Clicking yes brings up the Dialogue box "syncing cannot be enabled on this computer".

I have followed the advice in the following links:

http://support.apple.com/kb/TS1627

http://support.apple.com/kb/HT1386

Multiple times over.

My system is running OS X Mountain Lion.

Can anyone help me?

James