What I discovered and want to share. One language does not translated well in a SEF url. What I got was component/oscommerce instead of /webshop/ or whatsoever. This was because of an old Joomla menu itemid in the Trashed and a same new menuitem in front.
Or the problem is the upgrade from joomla 2.5 to 3.0? I don\'t know. But MarvikShop takes the oldest itemid, also in the trashed. After empty the menu trashed the problem was over and a nice SEF url shows up!