I fixed the issue by editing the selectrecipients.php page. I changed the following code:
<?php
if (isset($_POST["GeneralEmail"])) // Trigger
{
header("Location: "."/admin/mail/campaign_general.php");
} ?>
to:
<?php
if (isset($_POST["GeneralEmail"])) // Trigger
{
header("Location: "."/admin/mail/campaign_general.php");
exit;
} ?>