FabricJS generating PNG in nodeJS, but sometimes images show as black rectangles. Cause and options to fix/handle?

Question

We generate PNGs from the canvas in nodeJS, but usually when there are many images, sometimes some of the images just display as black rectangles. We assume those images timeout and so the black rectangles are there.

What is the cause? What are the timeouts (nodejs, express, fabricjs,... ) and values and configurations? What are the best options and workarounds? Thanks!