You entered an invitee that wasn't recognized by the server

I am running Mac 10.6.7 and iCal 4.0.4. I have an 2010 Exchange account configured in iCal ,and Mac Mail, it has been running fine for over a year now. Recently i have been getting an iCal which says

"Calandar entry name" had invitees not recognized by the server.

You entered an invitee that wasn't recognized by the server

Fix later Edit Event

If i click Fix Later the error pops up again in a few minutes.

If i click Edit Event it takes me to a specific week in the past but it doesn't open a calendar entry to be editied

I have Deleted and re-created the iCal account but i still get the error message

I have deleted, through an Outlook calendar, the calendar entry with the correct name in the week that i am taken to but i still get the error message

I get the following error message in Console

21/04/2011 08:52:38iCal[708] error = Error Domain=CalExchangeErrorDomain Code=-19 UserInfo=0x10e1900e0 "“Calandar entry name” had invitees not recognized by the server."

This is getting so irratating that i am considering using Mac Office Outlook. Can anyone help please.