WLAN doesn't work on Raspi3

I used current Volumio (3.631) on a Raspi3 for some months using LAN for network access. Now I wanted to change that and use WLAN instead. Unfortunately, it doesn’t connect to my WLAN network (translated from german to english):

  • Settings → Network: State: empty
  • Cable: Automatic IP: on
  • WLAN: on, automatic IP: on

My networks are listed correctly and if I activate the hotspot option I can connect to the Raspi. But if I select my WLAN network (password is there) and select Connect:

  • a message is shown like: WLAN network restarted…, after some seconds the network settings windows is refreshed, but networking state stays empty.

What can be the reason and what can I do to find out what’s happening here?

Thank you.

Strange, my WLAN AP is listed, but doesn’t connect if it’s at channel 11; channel 3 works. Channel 4 worked once, now it doesn’t… /etc/default/crda is REGDOMAIN=CH which is where I’m located.