I have a Nokia 925 WP and can´t establish a VPN-connection to my Fritz!Box, how can I get them connected?

Settings at Fritz!Box:
MyFritz! is activated, my Useraccount for Access from Internet too, VPN-Settings OK

Settings on my WP:
Servername from Fritz!Box VPN-Settings, VPN over L2TP/IPSec, Authentification Username+Password+PSK from Fritz!Box VPN-Settings

I have never been able to use a vpn reliably on wp.