This error message means that the client does not trust the server cert during the SSL handshake. Some browsers are a little more forgiving and give you a "Red bar", but calling from code will result in a 401 and a rejected call.

You do not need to do anything with the client cert settings in IIS (because I assume you are not using a client cert).

This exception message is telling you that the self-signed cert chain is rejected when validating on the client. You might be about to get around this by exporting (without the private key), the self-signed cert and installing it on the client machine as a root cert.

If this doesn't work, you need to make a new CA (certificate authority cert) and then generate a new server cert that is signed with the CA. This CA would finally have to be installed on the client machine as a root cert.

This is a good post that shows the process using makecert and pvk2pfx.

This error message means that the client does not trust the server cert during the SSL handshake. Some browsers are a little more forgiving and give you a "Red bar", but calling from code will result in a 401 and a rejected call.

You do not need to do anything with the client cert settings in IIS (because I assume you are not using a client cert).

This exception message is telling you that the self-signed cert chain is rejected when validating on the client. You might be about to get around this by exporting (without the private key), the self-signed cert and installing it on the client machine as a root cert.

If this doesn't work, you need to make a new CA (certificate authority cert) and then generate a new server cert that is signed with the CA. This CA would finally have to be installed on the client machine as a root cert.

This is a good post that shows the process using makecert and pvk2pfx.

EDIT: It looks like there might be a way to configure HttpClient to ignore SSL errors. But, I would strongly suggest that you try to not to have SSL errors from the start.