It might be misreading your "null" as a negative number - epoch time is measured as the number of seconds from 1.1.1970 IIRC, and a lot of time values are stored in the database in epoch time values. I'd check the code but I'M ON VACATION! so maybe Aaron'll get bored later today and tell you.
Looks like there was a problem with the code resetting everyone's posts-per-page count to 20 no matter what you'd set it to (like, it was 40 for you). So, for people who never moved it from 20, they wouldn't notice.