Hi,
I have the same Problem, but i am not that deep into Linux 
I run Volumio in the bedroom (hostname “bed”) an one with a Screen in the livingroom (hostname “screen”. (just call it by hostnames bed/screen)
Everytime i am on the “screen” and i click the multiroomdevice “bed” it works fine. when i want to go back to “screen” i see the progressbar running and the loading circle on the dimmed background.
only solution is to restart the “screen” to get the UI back.
bed:
RP1 with HiFiBerry DAC
Plugins:
- Spotify
screen:
RP3 with RP 7" Display
Plugins:
- Spotify
- Touch Display
Tried Solutions:
- reflashing both RP with the latest Volumio 2.502 (even without any Plugin) --> same Problem
- Using different Web Browsers (IE, Firefox, Safari, Chrome) --> same Problem
I hope there is a Solution out there.