Zend certified PHP/Magento developer

PayPal Express mail template not found?

Magento 2.2.3

I am using PayPal Express and the customer gets a confirmation mail with the following content:

We are sorry, an error has occured while generating this content.

This is coming from /vendor/magento/module-email/Model/Template/Filter.php

if ($this->_appState->getMode() == MagentoFrameworkAppState::MODE_DEVELOPER) {
    $value = sprintf(__('Error filtering template: %s'), $e->getMessage());
} else {
    $value = __("We're sorry, an error has occurred while generating this content.");
}

The order goes through, but the mail content is missing. Any idea where to start looking for the problem?

Thanks!