Hi Reed,
Don't know why for not downloading however there are some work around.
http://download.adobe.com/pub/adobe/coldfusion/hotfix_015.jar
You could try manually download (link above) update15 then place JAR in ColdFusion10\cfusion\hf-updates and run update from within CFadmin.
Since your other CF10 can download you could copy hotfix_015.jar from those.
From CF10 fresh install to update15 you might be better off to apply update15 manually:
-CMD prompt as administrator
-ColdFusion10\jre\bin>java -jar ColdFusion10\cfusion\hf-updates\hotfix_015.jar
-follow the graphic interface
Remember some updates increment the IIS connector so may need to run ColdFusion10\cfusion\runtime\bin\wsconfig.exe as administrator to apply new isapi_redirect.dll. Be mindful if workers.properties has customised settings when using wsconfig.exe tool.
HTH, Carl.