Just spent some hours to find out why NPrinting On Demmand stopped working for one of our clients.
It turned out that their certificate used for SSL was updated. And since the 9000/9001 ports used by On Demand must be assigned to a correct certificate, the On Demand stopped working (and it was hard finding out why, no log entries anywhere).
The issue can be solved by three steps (in a console prompt with Admin permissions):
1) Show existing configuration:
netsh http show sslcert
Take a note of the app id connected to port 9000/9001.
2) Delete the old configuration (example for port 9001):
netsh http delete sslcert ipport=0.0.0.0:9001
3) Add the new, correct configuration (again, here for port 9001):
netsh http add sslcert ipport=0.0.0.0:9001 certhash=<hash for new certificate> appid=<from 1 above>
Restart the NPrinting Service and you are done!
Hope it saved you some time, br Patrik