Edit cart bill to

[chaldama]

I currently have 3 webshops running in Joomla 1.7/Virtuemart 2.0 which are nearly finished.
I now encounter a problem.
At http://www.schoonheidsspeshitpillte-beverwijk.nl/webshop (please login with user name test and password test) it is possible during check out to alter or add a delivery address.

At both other sites, for example this one: http://www.werkstoelen.eu/ the link in the shoping cart does not work and results in an internal server error. All sites run at the same host, same server.

I notice that the URL structure is slightly different. The site where it does work has the suffix /user/edit_cart_bill_to
The sites where it does not work have /edit_cart_bill_to. So they miss the piece "/user". When I manually add this piece, it still does not work however.

Does anyone have an idea what goes wrong and how I can solve it?