I was working on email functionality for a Magento client and I faced this error.

If you observe above screenshot carefully then you will find that special characters are not looking the way they suppose to be.

I dug further and realized that ‘Magento core Dotmailer plugin’ was installed and it was decoding utf-8 string to a terrible creature by utf8_decode() function.

So, It was necessary for me to disable private function getProcessedTemplatePreviewAndOther() or update module version because in Magento 2.3.0 latest version it is solved and module version is updated.

Here, I’m sharing with you that how to update dotmailer plugin using composer:

composer require dotmailer/dotmailer-magento2-extension:"[Dotmailer Latest Version] as [Dotmailer Current Use Version]"

Like:

composer require dotmailer/dotmailer-magento2-extension:"3.0.3 as 2.6.0"

Note: Here 3.0.3 is dotmailer plugin version, which is used in our latest Magento 2.3.0. Aparts, 2.6.0 is dotmailer plugin version which is used in our current Magento 2.2.6.