Mail / iCloud rejects password on one mac but not the other

I have just upgraded my MacbookPro and my MacPro from 10.6 to 10.7.

For whatever reason - the MBP works fine and can access all my email accounts, whilst the MP is constantly rejecting the password for iCloud.

My Mobileme account was upgraded to iCloud several months ago and has been in daily use on both macs ever since.

The only difference in the upgrades is that the MBP upgrade was not a clean install, whereas the MP was installed with a clean hard drive and then all apps/data was transfered over.

Disk permissions have been repaired and that has made no difference.

Both macs are on the same network, with the same passwords and same account settings. Any suggestion how i can resolve this problem?

adam