"Your network settings have been changed by another application"

*I found the solution, at least for me* - I'm on OS 10.4.11 - The problem went away after I updated the firmware on my Airport Express.
Sorry, i don't have the link handy, but it's easy enough to find.

I also experienced this problem after doing the 2008-008 security (and also 10.4.11) updates. The solution as suggested above worked for me: Click ok then immediately click the lock to prevent further changes. Force quit on system preferences and then re-open. Problem solved. I also clicked the lock to see if the problem recurred and it didn't. Very simple fix.

Scrap that... it came back again after i unlocked and quit. Seems to work ok if I just lock it... but what if I want to make changes to the network??

This solution worked for me

Here is a 4-step solution that worked for me:
1) As soon as you get the looping message, Force Quit the System Preferences application.
2) Re-launch the System Preferences application and open the Sharing pane ( not the Network pane).
3) Check and if necessary restore your sharing settings (from your back up records), then close the Sharing pane and open the Network pane. The looping should now be gone
4) From the Location tab of the Network pane, choose "Edit Locations…". Duplicate your active Location, call the duplicate say, "Location Copy 1" then delete the original.

This has (so far) fixed the problem on both my computers.

You are absolutely brilliant! These steps resolved the issue immediately. Thanks so much for posting the information.

Unfortunately, the solution that I suggested only works for me until the next re-start, when all the Sharing settings are unlocked and corrupted again. When I correct the Sharing settings I find that Internet sharing spontaneously turns off the moment I leave that Internet tab. The only way to beat it is to first establish an Airport network with another computer; only then can you lock Internet Sharing "On", and have it remain "On" after you disconnect the network. Establishing an Ethernet network or a Firewire network will not do the trick.
As you can imagine I am heartily sick of this bug, which I have suffered for at least six months. The newest Security Update for PowerPC hasn't fixed it either.

I don't know if this will work for other Mac models, but I had a Mac Genius solve this one for me & it ended up not only fixing the "Network Settings" issue, but it also freed up memory and made my 4-year-old PowerBook G4 run like new. In this procedure, you trash your preferences, which doesn't delete your data, but you may need to set up Mail again, so it's a good idea to jot down your Mail account information. Also, I believe it'll delete all 'locations' other than 'automatic' in your Network Settings.

1. Quit all open apps
2. Repair Permissions
3. Drag the entire contents of your preference folder to the trash (MacintoshHD/Library/Preferences)
4. Empty Trash
5. Perform an SMC Reset (PowerBook G4: Shut down your computer. Press the power button and hold it down until it beeps, 5 - 10 seconds) http://support.apple.com/kb/HT1411
6. Perform a PRAM Reset (Restart your machine holding down Command Option+PR, wait for two or three chimes & release)

And now, a warning: (and NOW a warning?!), The next time you fire up any application, they'll act as though it's the first time you've ever run them. You'll have to tell iTunes where your music library is located (in your iTunes folder, btw) and if you use Quicken, you'll have to tell it to open your latest backup of it. Don't Panic. All your data is there. You just might need to tell some apps where to find it. I hope this helps!

This worked perfectly for me also.

And as mentioned above, it failed again after restarting preferences.

Roger.
Do you know if the "your network settings have been changed by another application" problem has any relation to my inability to log into "ftp" sites to upload or not being able to download apps from web sites?

It is a easy solution. Please install the latest security update 2009-002. And when you open the networks window, you will get the message "Your network settings have been changed by another application." Use you 'enter' key and the message vanishes for a second. In that second gap, click on 'Assist me...' at the bottom with your mouse. And then you get a message from Assist me. Press 'Cancel'. It should resolve.

Hope it helps!

Unfortunately Security Update 2009-002 makes absolutely no difference. I have suffered this maddening bug for 12 months now. As well I have suffered from the cryptic message "your modem has unexpectedly hung up" since the year 2003 - that's 6 years so help me!

Found this solution for Mac OS 10.4.11 while googling the web:
[A simpler fix I found tonight is to open System Preferences, go to Security and check "Require password to unlock each secure system preference."]
source: Pieter B. http://urbantektonic.com/tekblog/2008/10/mac-network-settings-changed/print/

My daughter just called with the "Your network settings have been changed by another application" message. We went through this fix. If you have multiple locations you will need to have a paper copy of all the settings for each location you have; 3x5 cards are wonderful for this. To begin restart with the Shift key down. When the computer restarts it will be in Safe Boot mode. Open the System Preferences. Open the Network preferences. Keep clicking OK to the message and get to Edit Locations menu. Create a new location. Name it what you like but do not duplicate a name; you can always go back and rename it. Then delete all the other locations. Restart the computer as you normally would. The Network settings are like new again with only the one you just created. You will have to go in and setup the Network settings manually. If you are using Airport you may have to turn Airport on again and go from there.