I want to download the JSON Info at
idHTTP := TIdHTTP.Create(nil);
JSON:=idHTTP.get('https://login.microsoftonline.com/common/.well-known/openid-configuration');
and get an error
10054 Connection reset by peer
. Downloading in the browser works.
How can I go about finding the issue?
10054 means the TCP connection is being reset abruptly on the remote end.
You are trying to download from an encrypted HTTPS url, so you need to assign a TIdSSLIOHandlerSocketBase-derived component (such as TIdSSLIOHandlerSocketOpenSSL) to the TIdHTTP.IOHandler property. But, you don't appear to be doing that.
If you are using an up-to-date version of Indy, TIdHTTP raises an EIdIOHandlerPropInvalid exception if an SSLIOHandler is not assigned when requesting an HTTPS url. But, if you are using an older version of Indy (which you did not indicate) that is not doing that check, then it is possible that TIdHTTP is creating the TCP connection and then the server resets the connection immediately when it doesn't receive a TLS handshake.
If you are using an up-to-date version of Indy, you can add the IdSSLOpenSSL unit in your uses clause and TIdHTTP will create a default TIdSSLIOHandlerSocketOpenSSL instance for you when needed, but just note that it will have only TLS 1.0 enabled on it, so if you need to use TLS 1.1+ instead then you will have to create the TIdSSLIOHandlerSocketOpenSSL object yourself and configure it as needed.