it's a reach but if you're using chrome try resetting all experimental features. I had issues with the on demand extension two weeks ago and this was the only thing that worked for me (I even reinstalled the whole NPrinting-Server).
I remember changing something in there a year ago, don't know what it was but after I pressed reset and closed / reopened chrome the extension worked without any further issues.
If you want more details on whats going on you can also open the developer console by pressing "F12" while you're on a the Qlik Sense sheet containing the On-Demand-Extension. You should see an error in the console and more details in the "Network"-tab.
In my case, the error response alternated between a 400 Bad Request (CORS-Header missing even though I added it in the admin panel) and a 403 Forbidden (a JSONresponse containing "user does not have permission to this resource" even though my user was an NPrinting administrator)