Although this is an old question, I worked around like this:
In components/modules/namecheap/apis/namecheap_api.php
// original code
if (!isset($args['ClientIP'])) {
$args['ClientIP'] = isset($_SERVER['REMOTE_ADDR']) ? $_SERVER['REMOTE_ADDR'] : '127.0.0.1';
}
// added this line:
// I used my own IPv4, but this worked as well (in another panel)
$args['ClientIP'] = '127.0.0.1';
And yes, namecheap still does not support IPv6.