When you double-click on the the sender of a message, the message only opens if the column “From” is large enough to display the hole name.
Same story if you double-click on the subject of a message.
You can bypass the problem by adjusting the column. But I should be nicer if you can just double-click also when the column is to small.
It is now fixed in our internal builds, it will be available in the next update (probably in a few days). You can workaround this issue to disable tooltips. Just go to Tools->Settings->Appearance and uncheck Use tooltips for overlapping texts.