Okay, so am I correct in understanding from your response that adding a new column to eCart shouldn't result in any changes at all to the confirmSSL.asp page? And if the other two pages are correct code-wise, then - in theory anyway - all should work well together?
The browser I am using is Firefox. Didn't try any other browsers as once I discovered that I couldn't quickly fix the error, I restored all the changed files on the server to their previous versions as it is a live site. And as such, there is not currently a URL for you to reproduce the problem.
As far as turning on the error message details, the web host would have to do that. I will badger them again tomorrow to do so but don't know if they will.
I am 100% positive I uploaded the cart definition files. I do wonder, however, if the browser was at fault somehow as - if the code is all correct as you say - I've found that both Firefox and Chrome can be terrible for using outdated files from their cache instead of checking for newer versions. I think they like to pretend that they are faster than they really are that way.
So, if all the code is correct, here is what I will do: I expect the site to be busy on Friday, so on Saturday I will try uploading all the changed files again and try testing it again. Hopefully. I'll be able to convince the host to turn on error reporting.
I will report back on Saturday as to what happens. Please let me know in the interim if you have any other thoughts.