“The operation failed. An object could not be found. Unable to..."

Hello to all,

I have a Outlook issue with one specific user (Secretary) & Outlook
calendaring.
Before when any user incl. the (Secretary) wanted to create a “New
Appointment� or “Meeting Request� what they would do is, go to calendar
select the day & hour, “invite attendees� send it & it was done.
From one day to the next we have a problem with the (Secretary) in the
“Invite Attendees� box, if we add the (Secretary) or a group in which she is
in, in the “Required� or “Optional� box there’s no problem but if we add the
(Secretary) or group in which she’s in, to the “Resources� box when we choose
“send� we get a message saying “The operation failed. An object could not be
found. Unable to directly book a resource for this meeting�. All the users
have Win XP Pro Eng with updates & SP2, Office 2003 with SP1,2 & updates,
we’ve deleted the user & created a new one with the same name, & nothing.
Please if anyone knows what could be the problem or if anyone has
experienced the same situation and managed to solve it, let me know.

Thanks to all.
Milton