294735 cannot order

A client of mine has a website www c o n t a c t l e n z e n o n l i n e dot be and he receives 1 or 2 emails each day from customers saying they cannot login and customers saying they cannot order. They claim they go on his site, they place an order, and when they register and confirm their order their shopping cart is empty. Normally the shopping cart should remember the order placed before registration. I have tested this myself and it is true. Then I had a programmer look at it and he said everything was OK. But my customer still gets those emails from his customers. The shopping cart was written a long time ago (> 5 years) and AFAICT it does not use cookies but sessions. We looked at this and this does not seem to be the issue. Any suggestions?