'CharlesS' correctly deduced that the problem was the setuid flag on the NetInfo Manager application itself. To correct the problem, open a terminal and type:
% cd /Applications/UtilitiesIf you're interested in reading more, the thread has details (courtesy of 'MervTormel') on exactly what the setuid bit is and why it's important to have it correctly set for NetInfo Manager.
% sudo chmod 4775 NetInfo\ Manager.app/Contents/MacOS/NetInfo\ Manager
Note: Edited to fix the backslash problem noted in the comments. Also note the article on StepWise referenced in the comments regarding security concerns.

