When a LPR printer does not respond within 15 minutes, as in out of paper or a jam, a popup message is sent to your print server. Of course, no one is logged on to receive it.

You can eliminate the popup by navigating to: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Print\Providers

Add Value name RetryPopup as a type REG_DWORD. Set the data value to:

0 - Disable the popup.
1 - Enable the popup.

This works in SP4 and later.