Just to confirm, that machine otherwise can access the internet with no problem? Are you able to access other internet links from ProSeries.. for example, if you go to Help -> Online -> Anything it launches a browser and brings up the sight? And you are able to sign in online?
To help with troubleshooting, Can you try the following?
1. Go to your ProSeries installation directory, and open \32bit\protax20.exe.config in Notepad or Word
2. Change<level value="INFO" /> to<level value="DEBUG" />, save and try updating again. This just generates more log information we can use to troubleshoot.
3. Mail the following files to firstname.lastname@example.org
Hopefully one of those files will give us a clue as to what is not working.