I have the same or a similar problem. If I select a "conversation" with more than one mail (e.g. a mail I send and one reply) kube starts to load my browser and fails with an error with the title "Error -- KIOExec" and text that starts with "File not found: data:text/html;charset=UTF-8,%3C%21DOCTYPE HTML PUBLIC %22-" and followed by the rest of the mail. In the kube window the first mail is shown correctly but the second one misses the text of the mail. The subject, sender and time are shown correctly.