Firefox/1.5.0.8:
I get the same error on my own website, and then I try to test on www.dnnsoft.com to avoid missing something. When I log in www.dnnsoft.com, and purchase something, it shows error like below image, the purchase can't be finished. And by the way, I can't add any product into my cart or buy any product directly too!
If you use IE6, it is no problem.