Hi @Grosen & @Dansko82
Welcome to the Sonos Community!
I highly recommend you turn off your WiFi router and Sonos speakers. Once the router has been off for at least 30 seconds, switch it back on and wait for an internet connection via WiFi before turning your speakers back on. Wait for solid white lights on your Sonos speakers before opening the Sonos app to try again.
It may even help to reboot your smartphone/tablet, or to try from another device.
If these steps don’t help you, I recommend you get in touch with our technical support team, who have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.