I'm unable to edit entries (clicking the "Edit entry" toolbar button doesn't have any apparent effect), and when I'm creating a new entry, a new (blank?) entry appears but I don't get to edit that entry. Also, when I exit JabRef after having added a new (blank?) entry without having saved the bib file, it doesn't ask me if I want to save it. However, if I manually save the file, the new blank entry will be there when I close and reopen the bib file, and if I delete it again, JabRef will ask me if I want to save the bib file upon closing it without having saved it already.
My OS is Windows XP and I'm running JabRef 2.8.1. I created the bib file with JabRef 2.8 and was able to work with it in both Windows XP and Windows 7, so apparently, there is something about the 2.8.1 version that causes this behavior.