Custom Error page isn't loading when URL contains "/ /"

I'm having a very specific issue with a site I'm building that is set up to use friendly URLs and custom error pages. I've been reading the message boards for days and although there are a lot of similar seeming posts, nothing addresses this problem specifically... at least to my newbie eyes.

Mostly everything on my site is running ok:

• URLs like "example.com/main/variable/variable" work just fine (redirects to example.com/index.php, loads main.php internally, and pulls the variables into an array)
• 404 errors are handled by index.php and work fine
• 403 errors when you try to access a subfolder with no index page mostly work fine too: "example.com/folder" shows the custom 403 error page at 403.php

Where I'm running into trouble is if the URL gets corrupted to contain a single space followed by a forward slash:

• "example.com/ /" or "example.com/folder/ /"
• "example.com/%20/" or "example.com/folder/%20/"

When this happens a 403 error gets triggered but it doesn't load the custom 403.php page.

Noteably, this problem does not occur if the URL lacks the trailing slash:

• "example.com/ " or "example.com/%20"
• "example.com/folder/ " or "example.com/folder/%20"

These all trigger 404 errors rather than 403, which is what I would like to happen when the trailing slash is included.

I realize this is a very specific situation that will only occur if someone mangles a url while they're typing it in, but I would love for it to not happen. I talked with my web host and they said everything is correct on their end.

Here's the code I'm currently running in .htaccess in my /public-html/ directory:

ErrorDocument 403 https://www.example.com/403.php

# Begin EnforceSSL 
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www.)?example.com$
RewriteCond %{HTTPS} !=on
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L]
</IfModule>
# End EnforceSSL

RewriteEngine On
RewriteBase /

# The Friendly URLs part
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?$1 [L,QSA,NC]

and here's the code in index.php which handles the friendly URLs:

<?php

// Create a URL array containing any passed variables, including requested page
$URL = explode("/", $_SERVER[QUERY_STRING]);

// Load the page
if (trim($URL[0]) == "index"){
    require_once("home.php");
} else if (trim(($URL[0]) == "")){
    require_once("home.php");           
} else if (file_exists(strtolower(trim($URL[0])).".php")) {
    require_once(strtolower(trim($URL[0])).".php");
} else {
    require_once("404.php");
}
?>

Any help is greatly appreciated!