[Zope] Where Is This Problem? Apache? Zope? SiteRoot??

[beno]

Hi;
This one is the strangest I've ever seen. I had another programmer develop 
a shopping cart for me built on EMarket. The site uses SiteRoot to proxy 
from Apache. Here's the problem as best I can describe it. If you go here:
http://blackbeardale.com/
click to enter the store, then click either of the two entries, then try 
and "Add" a product, you get this error:

Zope Error
Zope has encountered an error while publishing this resource.
Error Type: TypeError
Error Value: unsupported operand types for +

No, correction, MAYBE you get this error. Maybe you don't. My programmer 
doesn't. My client does. Now, we have this same shopping cart installed on 
another client's site and it works just fine. We tried a number of things 
and ended up renaming folder and changing SiteRoots such that a working URL 
pointed to the problem site and viola! the site worked fine. So it appeared 
there was a problem in the proxy. Now, I'd done something different with 
this proxying: I'd pointed two URLs to the same SiteRoot, so I ripped one 
of them out of my httpd.conf file, rebooted Apache, reinstalled SiteRoot, 
and STILL it doesn't work. But it works fine with any other URL. What gives?
beno
RH7.3, Zope2.5.1, what else ya need?