The logic appears sound and updates all look correct. What browser are you using? Is there a url where I can reproduce the problem?
You should be able to turn on error message details in both your browser and server to get more information about the actual error:
Another, though slower, solution is to remove chunks of code one at a time and narrow down where the error occurs.
If you use:
Response.Write "made it"
and move that down the page. You can narrow down where the code is errors because eventually it won't make it to the line of code, representing the fact that the error occurs above. Then move the code into the code chunk to eventually narrow down the exact line of code where the error occurs. Once you know the line of code it will be much easier diagnosing why.
Is it possible that you haven't uploaded the new updated cart definition files to the secure server after adding the column, leaving it out of sync?