Thanks for everyone's help with this so far. We really appreciate it! Unfortunately, the problem continues!!!
The solution TR proposed worked for a month or 2 and then our checkout stopped working again. People using many browsers on many platforms report issues on the checkout page on some or all of the javascript related regions of it. Myself, I find the problem mostly on the order total calculation and the shipping quote.
This problem persisted across multiple upgrades of the OS -- Mac OS X Server 10.5 with all the latest javascript updates installed as they were released. We just recently updated to 10.6 and the problem is the same.
I installed Firebug as TR suggested and watched the error reporting. And the two regions I had problems with showed the errors below:
POST http://somatics.org/?q=taxes/calculate 500 Internal Server Error 1.43s jquery.js (line 11)
POST http://somatics.org/?q=cart/checkout/shipping/quote 500 Internal Server Error 1.18s jquery.js (line 11)
I have looked all over the web for a newer version of JQuery than 1.2.6, including on the Drupal website and on the JSUpdate module page: All version on that page for Drupal 5, including the dev version, are 1.2.6. I even tried the file from the JSUpdate for Drupal 6, and that _reeeeally_ didn't work.
I don't know what to do. I don't know what else I can update. Our company has invested all our resources over the past few years into running our store on Ubercart, and our business is basically paralyzed now for an extended period of time. And each time I think we've fixed it, we're back where we started.
Does anyone know how I can fix this?