Kill Apple Remote Desktop via ssh
I tried to remote control my work machine from home using Apple Remote Desktop and got the following error:
Apple Remote Desktop or another administration application is currently running.
The problem was that I had left the same Apple Remote Desktop client running on my work machine. To solve this I
sshed to that machine and issued:
killall "Remote Desktop"
Update: You can then use remote desktop to reopen the Apple Remote Desktop client and go to preferences > Security and check “Allow control of this computer when this application is running”