I wrote an answer regarding this problem in the SEB forum. I mentioned that there is a known bug in SEB 2.1 for Mac OS X which causes that sometimes no browser user agent string is sent (or maybe a user agent just containing "SEB 2.1" and no browser information).
I wonder if a Moodle quiz could react to a missing or incomplete user agent string in the way Danny described it (clicking on a question number in the navigation block makes the "Next" button disappear and information entered in that question is lost)?
The user agent bug should be fixed in the SEB 2.1.1 testing version which can be found here.