I'm having the same problem and working with Contribute support to resolve it. My computer is a new Mac Intel machine. Contribute still works on my old computer. Anyway, I'll paste in her last email on solving the problem. BTW, so far I haven't been able to fix it.

"When the error msg "Another instance .. " pops up?
Under "user/Library/Application Support/Adobe/Contribute CS4/Configuration/IBETemp" a file "draftworkflow.lck" is created during the Publish/Send activity. The activity can be from Contribute application or from the Contribute's "Edit" plugin in Firefox. If there is a simultaneous activity then error msg "Another instance ..." error pops up.

Try the following :-
1. If the error msg "Another instance ..." pops up click ok. Do you still get a blank page? If not publish and try editing another page and publish.
Check if the page is published successfully.
2. Try editing the same page in Firefox browser. Quit Contribute and Relaunch it. Browse to the web page in Firefox and click "Edit". Update the page and publish.

Does any error msg pop up?

Then there could be a corruption issue please try the following :-

1. Uninstall Contribute.
2. Clean up the Preferences from :-
(a) Go to "user/Library/Application Support/Adobe/" remove
"Contribute CS4".
(b) Go to "user/Library/Preferences" and verify the file "Adobe Contribute 5 Preferences" does not exist.
(c) If you are using CS3, then delete "Adobe Contribute 4.1 Preferences"
and "Contribute CS3" folder also.
3. Install Contribute. (Do not launch)
4. Go to "Applications/Utilities/Adobe Utilities/Adobe Updater 6" Click on the "Adobe Updater" and install the updates for Contribute CS4.
5. Launch Contribute CS4 and connect to the web site.
6. Quit and relaunch Contribute.
7. Edit the pages and publish.
8. Browse to other pages in the website in Firefox browser and click on "Edit". Add the changes and publish. "