I've been spending all day figuring this out as well. I switched to Faraday (from vanilla Net::HTTP) and i get the proper responses now. Not sure if this is an option for you. But it got me going again.
The reason why it's going wrong is because (at least for me) the Net::HTTP client was constructing a faulty URI to send the request to. I saw this by installing the httplog gem to verify the requests. and i saw something along these lines: (Notice the double https://graph.... in the POST request)
D, [2021-04-07T17:42:09.050306 #2373045] DEBUG -- : [httplog] Connecting: graph.microsoft.com:443
D, [2021-04-07T17:42:09.225682 #2373045] DEBUG -- : [httplog] Sending: POST http://graph.microsoft.com:443https://graph.microsoft.com/v1.0/users/<....>/onlineMeetings