If receiving a HRR, and the checks in "4.2.8. Key Share" fail:

   Upon receipt of this extension in a HelloRetryRequest, the client
   MUST verify that (1) the selected_group field corresponds to a group
   which was provided in the "supported_groups" extension in the
   original ClientHello and (2) the selected_group field does not
   correspond to a group which was provided in the "key_share" extension
   in the original ClientHello. If either of these checks fails, then
   the client MUST abort the handshake with an "illegal_parameter"
   alert.

It looks like we're sending:

            NamedGroup serverGroup = NamedGroup.valueOf(spec.selectedGroup);
            if (serverGroup == null) {
                throw chc.conContext.fatal(Alert.UNEXPECTED_MESSAGE,
                        "Unsupported HelloRetryRequest selected group: " +
                                NamedGroup.nameOf(spec.selectedGroup));
            }

There might be two issues: the alert should be illegal_parameter, and no check for selected_group field with the ClientHello request.