I had the same problem upgrading from version 4 to 5 on Mac OS X 10.6. Here's what I did what seems to work so far.
Deleted the following in user home directory:
~/Library/Preferences/com.CounterPath.X_Lite_4.plist (General settings of version 4)
~/Library/Preferences/com.CounterPath.X_Lite.plist (General settings of version 5)
~/Library/Preferences/Counterpath Corporation/X-Lite 4 (Settings, history, etc. of version 4)
~/Library/Preferences/Counterpath Corporation/X-Lite (Settings, history, etc. of version 5)
This will however remove all settings, contacts, call lists, etc. I believe. So best to make a backup of those files.