"CalendarAgent wants to use your confidential information stored in "Exchange Access Group" in your keychain."

Since installing Mountain Lion I keep getting the message "CalendarAgent wants to use your confidential information stored in "Exchange Access Group" in your keychain." Even when I accept allow, it keeps prompting, let alone when I deny it. It is super-irritating. What happened?

I have cleaned my machine, and did disk repair from my Disk Utility. The problem still persists.

Advice, pls.