add to cart and checkout issues when not logged in

[Buddha160]

I'm experiencing a real issue with VirtueMart (version I'm using is 3.0.16). Whenever I put something into the cart after i clear cache, I am no longer able to modify the cart until I clear server side cache again. It says that it's modified (changed quantities and/or delete item), but when I click continue shopping or home and try to add another item or go back to the shopping cart it shows only the original unmodified item. This only happens when one is not logged in to the site. I have tried turning off SEF, disabling the SSL and just about anything that I can find and so far nothing works.

website is https://www.cerrigspeir.com/

can anyone help?

[GJC Web Design]

until I clear server side cache again

well... doesn't this explain it...  switch off this caching