Hi Zvonimir,
I have checked my local environment but didn’t find the Notice showing on your website. It might be something on your website which cause this. You can ignore the Notice by adding the define( ‘WP_DEBUG’, false ); into the wp-config.php. If you still want to investigate it, I would like to suggest troubleshooting the following common issue to help us to recognize the reason.
1.Theme Related Issue
2.Plugin Conflict
3.Theme Update related issue
4.Child Theme Related issue
5.CSS/JS Customization
6.Disabling Cache
7.Disabling CDN
If you discover that an issue is coming from a custom code or 3rd party plugin, kindly consult with a developer or contact the plugin author. Please note that we do not provide support for custom codes and 3rd party plugins.
If none of this helps, please copy your live site to a staging server so we could troubleshoot freely without breaking your live site.
And give us access to the secure note, including:
– WordPress Site URL & Login URL
– WordPress Admin username/password
To create a secure note, click the key icon underneath any of your posts.
Thanks