Internal Server Error while sending a loop of emails

Hello, good afternoon.

We have encountered an error that behaves as follows:
Once we send emails (several or a lot, it doesn't matter the amount) the server sends them rightly.
After this, it's supposed to redirect to a php page (line 306 within the php file "fss_mailing_enviar.php" which we have attached):

header("Location: ".$GoToPage);

where $GoToPage stores the value "css_mailing_enviar_ok_resumen.php".

However, after several seconds, we get and error window with a 500 error (Internal Server Error, like the one you can view int attached image within this email). So the problem is not connected with sending emails but with redirecting to this page, due to an error connected with the server.

We'll appreciate you help.
Thanks in advance.