Not able to add calendar item

[Travis]

When trying to add an item to users calendar, they get a message:

'The item could not be saved to this folder. The folder has been delted,
moved, or you do not have permission. Do you want to save a copy of it in the
default folder for the item?'

If the user chooses yes they get:

'You do not have sufficient permission to perform this operation on this
object. See the folder contact of your system administrator.'

Well, being the system admin, I checked the permissions to the calendar and
made sure that the user had owner permission for the calendar. This issue
started happening two days ago and no changes have been made to the user's
permissions.

We are using Exchange 2000 with the latest patches and service packs and the
user is using Outlook 2000 with the latest patches and service packs.

Any suggestions would be really appreciated.

Thanks