"Train as Good" from spam box erases email headers

When I Train as Good from the spam box, the message moves from the spam box to the inbox but once there Subject, From, Date, etc are all completely blank. Very annoying, and makes the plugin unusable.

Any ideas as to what is causing this?

This is not due to the plug-in; it simply asks Mail to move the message. My guess is that your Mail database is damaged. You could quit Mail and delete the file:

/Users/<username>/Library/Mail/Envelope Index

Then it will build a new database for you.