sorry for the delay. I asked the network team for the remedy...
I think in very simple terms, the people picker make a service
call whenever you click ‘check names’ or ‘bring up the address book’... that
call goes to some back-end SP web services but that call should be over HTTPS
not HTTP
That error message is really saying you reached a HTTPS endpoint
(designed to serve HTTPS requests) but your request is coming in as HTTP.

For us:
My best guess is - it is a MS issue (so maybe something similar
to diagram above
Extra
FYI – this link https://support.microsoft.com/en-us/help/2901624/people-picker-field-embedded-in-office-documents-does-not-show-add-peo