Hi,
this usually works, so you should investigate more about it on your environment.
The request you highligh notifies that the PDF has been genereated correctly and indicates the URL.
Then MapStore should show the preview but somehow at that stage it can not load it.
It could be several things: the pdf is not reachable for some reason, or it is corrupted so the preview is not working...
This are the first things that comes in my mind but to see what is effectively affecting your environment, you need more information.
What if you open the URL in the response JSON?
Do you see any error in MapStore logs? Any error in browser console?