This bug is with Evolution 2.6.1 in Dapper.
On a few occasions I have found that the message display window has frozen, even though other parts of Evolution appear to be working. In other words I can select other messages in the message list, or go to other folders and select messages there. And if the messages are unread they will be marked as read when I select them. However the message window will appear to be stuck on the same message and won't move.
Last time it happened I tried to close Evolution from the file menu, but all that happened was that the Evolution main window (the only one I had open at the time) got greyed out yet would not close. Clicking on the window manager close-window button did not work either. However running "evolution --force-shutdown" did work and the window finally closed.
Thank you for your bug. Do you still have that problem? Do you have any error on the command line if you start evolution from there? What is evolution doing according to the status bur when that's happening?