outlook errors

This is the situation when a couple of workers sends the frontdesk operator meeting request she can't open some and some she can she is unable to anything with it eg.delete, move zilch..Even whe some one sends an update to the meeting she unable to view it the error message is "operation faild, An object cannot be found" and if you try to or open it "unable to open meeting, the object could not be found.

any suggestions.