Entering a URL in form triggers 404 Error (No reserved names used)

This is a weird one. If a visitor enters plain text in a form, the form submits without error. If the visitor enters text beginning with “https://”, the form post triggers a 404 error. The content of the field shouldn’t matter because it is treated as plain text and not a URL for redirection. I was able to recreate the problem with the minimal code below.

As expected, if the user selects “aaaaaaa”, the form does nothing. If he selects “https://”, the wordpress 404 page comes up. The file is in its own subdirectory and submits to itself. No processing at all.

This is the entire file:

<?php
// No processing or redirecting at all
?>

A form containing a value that begins with "https://", "https://" or "ftp://" causes a 404 error.<br/>
<form method="post" action="">
    <select name="uniquename182514">
        <option value="aaaaaaa">aaaaaaa</option>
        <option value="https://">https://</option>
    </select>
    <input type="Submit" value="Send">
</form>

This error started happening anywhere from 3 days ago to possibly a few weeks ago without any changes in my own code. It happens even when all plugins are disabled. The same code works fine on two other servers with similar setups. Could this be a problem with the server configuration? It is a shared Linux server hosted by GoDaddy.