Here’s another OS X Tiger to Leopard upgrade issue…
From time to time while on the road (in the train rather…) I connect to the Internet through my Nokia mobile phone hooked up to the MacBook via Bluetooth. This stopped working with Leopard. The phone icon in the menu bar showed “Connecting” for a few seconds and switched to “Disconnecting” immediately afterwards. The Console listed as error:
MPPE required, but MS-CHAP[v2] auth not performed.
A Google search directed me to a Mac OS X Hints article that explains that you needed to change CCPEnabled from 1 to 0 in /Library/Preferences/SystemConfiguration/preferences.plist – and restart the system, that’s important.