Hi Xavier,
I think it is a server specific problem. Because I did a test with a clean Joomla install without any other extension.
The server where the problem exists, belongs to a very reputable hosting company. The greatest Joomla hoster of the Netherlands. Very expensive but also very fast and gives very high quality. The companyname is:
www.byte.com
But for some reason they interact differently with url_rewriting which is used in Hikashop as URL rewriting for Joomla is turned on.
When I made a copy of this website and put it to another server by another hoster everything works fine.
Byte runs PHP in CGI mode. Maybe it is useful to know
I hope this will help you by finding a solution for this problem.
With Kind regards,
Lumiga