JavaScript alert on sidebar

Hello,
I hope someone can help me, I’m sure this has a very simple explanation but I have been trinity to resolved this problem but I can’t find out what is wrong.

I have a call to a php file in my sidebar.
<?php include(“newsletter-signup.php”); ?>
this file is in the theme folder with the rest of the files.

The content of this file is this:

[Code moderated as per the Forum Rules. Please use the pastebin]

I removed the content of the php section to leave only what is giving me the problem. when I click on the submit button, the alert shows up but the page under it goes to a 404 error page, and when I click the ok button on the alert box it stays on that page. I know I can call the history function of js but I don’t want the 404 error to show up, why the alert doesn’t show on the current page?
thanks.