It's back working for me now, about 2.5 days after the problem first
started. In case it helps anyone. I had, by now, unlinked the Mi Home
app, then, linked it again. (It didn't help earlier, with Google Home
app still saying Mi Home could not be rea...
Sounds like pretty much a rather large global problem. I understand
problems can happen... but that this seems to be taking quite a long
time to rectify does not look good.