to me it looks like that after hours in operation and when the IP address expired the Raspi is not requesting a new IP address ( or the old one again )
The effect is that volumio is not reachable at all … not from the browser, not the app, it just does not exist anymore in the LAN.
I have to mention this is an effect I observed on my WLAN, I did not test wired yet.
I now tried to set the lease time on the server to “infinite” … will let you know what happens
I am running the box on version 1.5