Why had Calendar stopped sync with iCloud?

I can't work this one out. MacPro (1 yr), iPad2, iPhone4 - all with latest updates. Everything working normally for many months.

Today, just with Calendar

• iCloud Safari web interface working fine
• iPhone4 continues to up/down sync fine with iCloud
• iPad can create and upsync to iCloud, but not downsync. System restart, no change. If I create an entry on the iPad and then delete it in iCloud, iCloud pushes the creation and deletion to the iPhone, but not to the iPad or the Mac.
• MacPro creates iCloud Calendar entries but cannot upsync or downsync. I notice that Cmd Refresh takes for ever. System restarted twice, no change.

Any ideas?