What happens if you delete the file org.apache.servicemix.bundles.commons-codec-1.3.0.jar from the userlib folder?
This file caused problems earlier with base64 encoded images in templates, see also issue #17:
If the problem reproducible, then it could be a request for the Mendix support. The module seemed to be supported by Mendix.
Put your base 64 values in your img element like the example below (replace the base64 values after the comma with yours), and your img element in your html body.
<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO 9TXL0Y4OHwAAAABJRU5ErkJggg==" alt="Red dot">
Thanks for sharing this HTML mail error code as it is also available in the w3school but unable to get that. printer offline fix assured me to get the template module.