Strangely my Essence still has the same firmware, also the Google. Cast version is the same.At least my M5 is much more stable now.
No dropouts so far.
And no "* Authentication server unreachable, stopping Google Cast!" messages so far.
As written before, Since last week (after the reboot) it started working again with Google Home, but very unstable.
After your message I did a reboot again and now it seems much more stable in the Google home group. And still the same firmware versions..