i connect to the web via LAN and IPs are assigned via the DHCP server. so a simple
sudo dhclient -r sudo dhclient
does not work. is there another way to get a new IP address from the DHCP server?
You should be able to be ifconfig /release and ifconfig /renew (the syntax might be a bit different I'm coming from Windows here) to drop the current IP and request a new IP from the DHCP server. However if your reservation is still valid the DHCP server will give you the same IP address.
What do you mean it does not work? Don't you get an IP address?
If you get one but this is the "old" one then this is totally OK. The server may be configured to hand out the same address according to the MAC (or other variables). It may even hand out the same address as it is the only free one in the DHCP range. Or it is lazy and sees no reason to hand out another one. An IP is an IP; they are all equal and it makes no difference which one you got.