Delegates sending Meeting request and their errrors

In our organization we have an admin assistant who sends outlook meeting
requests on behalf of her boss. Most of the time it works fine, however
sometimes specific users will get error messages when attempting to accept
the requests. One of the error messages states that the action can not be
performed because there is no organizer, while the other message states that
the meeting request is out of date.

I believe this to be a possible setting on the recipient's PC. Anything you
can suggest?

Thanks a bunch,