Thanks for your reply, but I don't think that this problem is router based. WiFi seems to be connected after wakeup from sleep, you can see the traffic symbol for incoming & outgoing traffic, WiFi says it is connected. But there is no network communication possible. Someone in this thread said that this problem also exists in the official CM 12.1 sources if I remember rigth.I was able to fix it with a setting in the router.
http://forum.xda-developers.com/showthread.php?p=63034491
