Got the solution for this today - corrupt XML file.
Instructions to correct that worked for us:
The error message is shown by NPrinting Server, normally when you click on the "Monitoring tool" option.
The problem is due to a corruption in the schedules.xml file.
This problem is due to a corrupted file.
i. Open NPrinting Server and stop the service.
ii. Open the C:\ProgramData\NPrinting\Monitor folder and remove the schedules.xml file.
iii. Restart the service.