url prefix wrong

[Ingo Jochim <i jochim querysoft de>]

 I modified my kickstart script from using DHCP to a fixed ip address.
From:
network --bootproto=dhcp --device=eth0
To:
network --device=eth0 --bootproto static --ip 192.168.52.171
    --netmask 255.255.255.0 --gateway 192.168.52.1
    --nameserver 192.168.52.1

With DHCP everything was working fine to download all files from a ftp 
server.
url --url ftp://192.168.52.1/OS_FC4

With a fixed ip address it try's to download the files from a wrong url.
It has an additional slash in the url.
ftp://192.168.52.1//OS_FC4

On the screen there is a message about url_prefix /OS_FC4.

Why is that not working now and how can I set the url_prefix manually?

Any ideas?
Ingo