Cannot respond to a meeting without an organizer

This is the situation where the error(s) occur: Exchange 2007 SP1 (build
240.6) and on the client side Outlook 2003 SP3 running on Windows XP SP3 and
some of the users have an Windows mobile phone (Windows mobile 6 or 6.1).

The error appears when the users accepts, refuse or alter the meeting
request in Outlook. When the same meeting request is opened in OWA there is
no error.

Is this a know bug? Is there a work around or a good solutions for this
problem?

I didn't found any usefull tips only a discussion on the Apple support forum
but there is nog much more info, only questions.

Any help or suggestions are appreciated.