When the webkit viewer declines to render then the message pane shows previously displayed HTML message. If you "Open message in new window" the popup is empty. What should happen is Claws should always show the current message - even if it is rendered as: plain text, HTML as text, or just a pane with error message as a last resort. If the current message is shown as plain text, then moving to an "unrenderable" HTML message displays the previous "valid" HTML message, whatever that happened to be. This is with Ubuntu Yakkety.
looks like another duplicate bug report *** This bug has been marked as a duplicate of bug 3644 ***