When a computer has ARD open, by default you cannot log into it using Remote Desktop from another host. To fix this, use the following command:
defaults write /Library/Preferences/com.apple.RemoteDesktop AdminConsoleAllowsRemoteControl -bool false
And then run the kickstart -restart -agent command from /System/Library/CoreServices/ARD Agent.app/Contents/Resources
/System/Library/CoreServices/ARD Agent.app/Contents/Resources/kickstart’ -restart -agent
