I can click on the gray box, because it contains the image URL. When I open it, I get this Error: ERROR 9504: Could not fetch the image - request timed out
I don’t think that this is necessary. If you open the workflow and click on “Test”, you can see the JSON code. In there you find the image URL. This is an URL to the image file with a signature. But this URL is public even if “Require Image and File URL Signing” is turned on.
As stated here, this URL signature doesn’t seem to have an expiration.
It seems that while creating the PDF file, the rendering service cannot access the image in some cases.