I was trying really hard to reproduce the problem. I installed version 1.92 again, created similar survey I had, upgraded LimeSurvey to 2.00 but the problem didn't occur. The survey was OK after the upgrade.
I am quite sure the problem occurred when LImeSurvey got upgraded. However, it wasn't me who did the upgrade so I can't be sure what really happened. Perhaps something went wrong with the upgrade and the database didn't get upgraded correctly.
Also, I am testing against Version 2.00+ Build 121117. Previously, when the problem occurred, the build number was smaller.