Connecting to Prosody via JAXL: "This server does not serve username"

Question

I have my own prosody server setup and can connect to it via a windows XMPP client. The server has two accounts on it, and I can connect as both of them and exchange messages between them (I am using the built-in client in Mozilla Thunderbird).

I have a Virtualhost configured on the server ("domain.com").

I am now trying to connect to the server via PHP as one of the accounts and send a message to the other. I have chosen JAXL to do that.

    $client = new JAXL(array(
        'jid' => 'username',
        'host' => 'domain.com',
        'pass' => 'mylongcomplicatedpassword',
        'auth_type' => 'SCRAM-SHA-1',
        'log_level' => JAXLLogger::DEBUG,
        'log_path' => './jaxl.log',
        'strict' => false,
    ));
    $client->start();

Note the format of 'jid'. This results in an error "This server does not serve username".

If I change the jid to [email protected], I get instead "Invalid username."

If I add

$client->require_xep(array('0114'));

before the start(), I get a "This server does not serve... ". in both formats of the JID.

What could be wrong? This is running on PHP 7.1 and the latest versions of JAXL and prosody.