I am having the same issue. It happens for single messages or messages grouped in a thread. Double clicking will only open a blank window with some action buttons. You can go to the individual emails in a thread in the main window by expanding the thread and selecting the individual email.
Also, just want to add my voice to the chorus of I will pay you more money to keep this plug-in alive and working. It is perhaps the single best plugin/app I use. Thank you for it.