Suddenly connect on my Marantz AVR SR5009 stop working.

I am (still) able to stream by Airplay, listen to inter radio, ect..., however when switching

over from the Ipad to the AVR I have three seconds of music and then it stops. This display indicates

waiting, connecting and then online...

The setup:

Cable modem in bridge modus to Netgear Nighthawk.

Fixid IP assigned by the router to the AVR and port forwarding for Spotify enabled.

On every apparatus spotify works just fine. According to the ISP everything is OK and working fine.

The Marantz indicates that the connection is ok, etc...

Who knows what's wrong and can solve he problem...?