When your saved design cannot be restored on the canvas, chances are high that it is too large to be handled in your browser. This happens when too many or too large external images are part of the design. The uploaded external files take up too much storage space and the Open Process breaks.
In order to avoid this, downsize and compress the external images before you upload them.
Example for a website to resize images: https://imageresizer.com/
Example for a website to compress images: https://compresspng.com/