That did it, thanks!
However, I think I've also stumbled across the real culprit. An extension called "Securitycheck" which I thought had been disabled but somehow became re-enabled. As I understood it, this extension just looked at your active extensions and advised as to whether there were updates available. This also has a plugin that adds "security", after I "fixed" the problem using your solution above, I re-enabled sh404SEF and all was going well until I went to checkout and then got a 400 error message indicating that I looked like a hacker "too many requests". I remember having this issue before and so when I went and looked at this plugin, I found it had been enabled, once again, disabling it and proper operation was restored. I even went back and "unfixed" the "cart" file and the problem seemed to be gone. Re-enabling Securitycheck and it came back.
Sorry for being a pain on this one but I hope what I discovered will help someone else.