I have sorted it out.
There is actually a typo on line 60 of forgot.php
.
For some reason I wrote single quotes. So, I replaced this:
$mail->SMTPSecure = 'PHPMailer::ENCRYPTION_SMTPS';
through this:
$mail->SMTPSecure = PHPMailer::ENCRYPTION_SMTPS;
Now, it works and I can send emails to reset my password.