Magento 2.2.5: Warning: Cannot modify header information - headers already sent by [custom_theme registration.php]

Question

I'm developing a custom theme and running into errors. The theme has been tested and working on various servers but, for some reason, since uploading to siteground I get the following error (file paths, vendor name etc. have been redacted and replaced with notes contained inside square brackets).

Warning: Cannot modify header information - headers already sent

"0":"Warning: Cannot modify header information - headers already sent by (output started at /[magento_root_directory]/app/design/frontend/[vendor]/[theme]/registration.php:1) in /[magento_root_directory]/vendor/magento/framework/Stdlib/Cookie/PhpCookieManager.php on line 148","1"

I've read that this is to do with whitespace before the <?php tag, but there is no whitespace.

My registration.php file is as follows (with vendor/theme name redacted):

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::THEME,
    'frontend/[VendorName]/[themename]',
    __DIR__
);

This is copied exactly from the file. The <?php tag is on the first line and there is no whitespace following it.

The site is installed on a siteground shared hosting instance, though I don't think this should make a difference.

Does anyone know what the problem is?

There is a massive stacktrace following the error message, which I can share if this information is not enough.

When I visit the website homepage I see a blank page, with nothing logged in the console and the URL reads:

http://[my_website].com/pub/errors/report.php?id=[error_id]&skin=default

The page reloads, looping indefinitely and creating a new report each time with the same error message.

Update

My theme.xml file is as follows:

<!--
/**
* Copyright © Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
-->
<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
    <title>[ThemeTitle]</title>
    <parent>Magento/blank</parent>
    <media>
        <preview_image>media/preview.jpg</preview_image>
    </media>
</theme>

Answer 1

Just check registration.php file on UTF-8 BOM elements. This a symbol on the start of file what can be seen without special mode in editors.