I'm having some issues using the symbolPreview for any fill symbols. it works fine for simple-marker, but if I use a simple-fill symbol, nothing renders and I get a 404 error for https://js.arcgis.com/4.6/esri/symbols/patterns/solid.png
Boris,
Try commenting out the following line: "style: solid" from the fill symbol. I was receiving the same display issue, but without the 404 error until I removed the style property.
thank you. that worked for me as well. But I can't understand why it worked.
and obviously deleting a property of the object after creating it is not optimal.