Since you're using v1.5.0 on your site, you need the /includes/modules/pages/password_forgotten/header_php.php file from either the original Zen Cart v1.5.0 code, or from a COWOA plugin properly designed for v1.5.0.
Using the v1.5.1 version of that file will cause the symptoms you posted.
Normally if one has a backup from before installing plugins, they can restore damaged files from that backup.
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