After upgrading the Apple Remote Desktop client on a number of machines, and using the new 3.9 version of the Remote Desktop app, some machines have started repeatedly displaying a dialog box (on the client machine, not on the controlling machine) saying ard agent wants to use the “PrivateKeyStore-501” keychain and prompting for a password. As there is no password for this keychain, there is no way to enter the correct password and hit OK. Your only other option is to keep hitting Cancel repeatedly – yet it keeps coming back.
Rebooting the affected machine will fix this, but that’s not so easy when it’s a server.
Dropping into Terminal instead and using kickstart to Restart the ARD Agent and helper seems to fix the issue as well.
sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart -restart -agent