Created attachment 110432 [details]
Swapped messages

When I first noticed the problem, the situation was as shown in the 2 screenshot pieces on the left side of the attached image, that is: selecting one entry in the message list (let's call it message A) showed the content of another email (message B); then, clicking on the message list entry seemingly representing message B actually showed message A. 

Then I restarted KMail and now it's similar but different, as showed in the 2 screenshot pieces on the right side of the attached image, that is: pairs of emails are threaded in wrong, swapped positions, but this time the content shown in the message pane belongs to the entry selected in the message list.

This happens with several pairs of emails.