Error when trying to send HTML email on event onActivateUser
Changing sendNotificationEmail() to this should fix it for you:

public function sendNotificationMail(UserModel $user) {
    $mailer = craft()-&gt;email;
    $mailToBeSent = new EmailModel();

    $mailToBeSent-&gt;subject = Craft::t('Nieuwe gebruiker geregistreerd');
    $mailToBeSent-&gt;fromName = 'xyz';
    $mailToBeSent-&gt;sender = 'noreply@xyz.be';

    $mailToBeSent-&gt;toEmail = 'xyz@xyz.be';

    $originalTemplateMode = craft()-&gt;templates-&gt;getTemplateMode();
    craft()-&gt;template-&gt;setTemplateMode(TemplateMode::Site);

    $content = craft()-&gt;templates-&gt;render(
        '_emails/customer-service-notification',
        [
            'userInfo' =&gt; [
                'email' =&gt; $user-&gt;email,
                'name' =&gt; $user-&gt;fullName,
            ],
        ]
    );

    $mailToBeSent-&gt;htmlBody = $content;
    $mailer-&gt;sendEmail($mailToBeSent);

    craft()-&gt;templates-&gt;setTemplateMode($originalTemplateMode);
}