WebUI hangs after switching to WIFI

I setup ALLO BOSS + PI3. Everything worked perfectly when PI3 was connected to ethernet with a cable. Then I wanted to switch PI3 to a wifi connection. I setup the wifi credentials (in /etc/network/interfaces) and I was able to access pi3/Volumio with SSH.
However opening WEBUI did not work any longer (the home page displays and immediately hangs forever).
Note that IOS and Android apps worked. It’s just WEBUI which does not work through the wifi connection.
I tried different thinks like removing completely the lan0 eth configuration from /etc/network/interfaces to leave only the Wlan0 setup. This did not help.
Any idea ?