I am having the same problem. I just upgraded JoomSEF to the newest version and installed the latest Virtuemart SEF extension, but I'm getting the same thing. I can add things to the cart, but when I click \"checkout\" it empties the cart can't go any further.

I have a dedicated SSL certificate.

My client really needs this fixed ASAP, any help highly appreciated.

I have the same problem. Has the creator replied to anyone to let us know what to do?

here's what worked for me... make sure your URL in the store information screen includes www . For instance, I had perfectmatchfavors.com which should of been http://www.perfectmatchfavors.com

Once I changed that it magically all worked. It was pure chance that I figured that out. Hopefully this will work for you and everyone else that is having this issue.