I also had the probem with volumio boxes not being found on Spotify.
This works at least for me:
1: ssh to volumio box
2: in the file /etc/hosts there are two entrys for spotify.com (mentioned earlier on this thread), remove or comment them out
3: disable and then re-enable spotify plugin
Now the volumio should reappear on Spotify, if not then
4: uninstall and reinstall the Spotify plugin
Hope this helps you too 
… I wonder what was the original idea with adding these ip addresses to /etc/hosts in first place?