User 2 sends User 1 a meeting appointment with both users being internal. User 1 accepts meeting and can send response. If User 2 then modifies the appointment and User 1 accepts they are unable to send an acceptance and Outlook comes up with "could not complete the operation one or more parameters are invalid"
If User 1 clicks accept and edit response we can manually put in the user and then send.
I have recreated Outlook Profile, flush suggested contacts, recreated nk2 file however it still doesn't work. and it only affects these 2 users.
Exchange 2010 SP3 and Outlook 2007 with latest SP and patches