Why are the meeting invitations i receive invalid?
If you get invited to meetings via Exchange, get an email containing a vCalendar/VCF file attachment, click on the attachment icon and get the error message saying “The item in the calendar is not valid. The message does contain a calendar, but the calendar contains no events, tasks or free/busy information.”, you should disable the “Itip Formatter” plugin by going to “Edit | Plugins”. After that, meeting invitations should display correctly.