Known bug,
covered here, on UBI site, and the Bugzilla site. A 'work around' is to open the messages up (early in the mission unless you want to watch paint dry) and set the dropdown at the top of the 'clipboard' to "sent messages".... that way you'll not see any messages, but will be able to open it in moments to check on mission status etc.
(You don't need to read messages - contact reports appear at map screen top anyway, and if a new mission comes in for you it'll show on the mission page of the clipboard).
For info - if you have bugs/queries try using the 'search' function first, to see if an existing thread sorts you out, rather than opening a new thread on what (for many) is an old, long discussed issue.... I'm not telling you off here, but others are less patient
