I just downloaded and installed RC 7.
For some messages, the content isn’t visible, in either the preview pane or when I open the message up in it’s own window.
That it’s only happening for some emails, but not all (I can’t discern a pattern for which ones this is happening to), leads me to believe that it may be a setting I’m missing, but I went through the program Settings and can’t find anything that seems to apply to this issue.
Below is a screenshot of a message that I know has text in it (some text and a link), as I viewed in v6 before installing RC7.