I use Apple Remote Desktop (ARD) to connect to my servers at work. I have another copy of ARD installed on one of the servers as well, and occasionally I’ll need to use ARD on my local machine to connect to a server remotely, and then run another instance of ARD on the server, all within the instance of it running on my desktop. If you’re having trouble following, that’s like calling someone and telling them to call other people for you.
There’s a security preference setting in ARD to disable you from doing exactly this, so as to prevent someone from controlling or viewing your screen when you are using ARD. Somehow, this setting managed to enable itself. You’ll be presented with a window stating “You are not authorized to control this computer. Apple Remote Desktop or another administration application is currently running.” This is highly irritating since it kills your remote session and the only way to fix it is to issue a killall “Remote Desktop” terminal command remotely to that machine, or to get physical access to the machine and quit ARD.
After a little digging in .plist files, I found the entry I was looking for. Right click (or control+click) on the Remote Desktop application and select ‘Show Package Contents.’ This will open a new Finder window. Open the ‘Contents’ folder, and then open the file called ‘Info.plist‘ Find the entry in this property list called “LSMultipleInstancesProhibited” and change its value to “No.” Save the file, relaunch ARD and you should be good to go.