Some data don’t support UTF-8

Hi,

Could you check what PHP version your website is running on?

PHP 7.0.22-0ubuntu0.16.04.1 (cli) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2017 Zend Technologies
    with Zend OPcache v7.0.22-0ubuntu0.16.04.1, Copyright (c) 1999-2017, by Zend Technologies

Strange, there could be issues with older PHP versions but with PHP7 this should work.

If you have a sandbox site where you can safely alter the code of plugins, you may try this: find this line in <plugin dir>/public/frontend.php
https://github.com/duracelltomi/gtm4wp/blob/master/public/frontend.php#L661

Remove this part:
, JSON_UNESCAPED_UNICODE

@duracelltomi
I try to remove JSON_UNESCAPED_UNICODE at https://github.com/duracelltomi/gtm4wp/blob/master/public/frontend.php#L683. It still is the same problem.