It's very likely that your PrintMonitor Preferences is set differently on the two systems. When the PrintMonitor window is active, select Preferences in the File menu. One of PrintMonitor's preferences is the ability to set:
"When a Manual feed job starts:"
Give no notification
Display icon in menu bar
Also display alert
The system giving you the additional dialog most likely has the third option selected, whereas the one not giving you the error most likely has the first option selected.