I rely on Screen Sharing to allow a machine running Leopard to display the screen of a second machine running Mountain Lion. While it’s occasionally a bit flaky, generally it works ok. However, after installing Remote Desktop Client Update 3.7.0 from Apple, it stopped working completely.
[Update: It turns out that the problems I was having were as a result of a bug in the 3.7.0 update. They were resolved completely by a later update from Apple. The fix below, sort of worked, but was unreliable. Since the bug fix from Apple Screen Sharing has been again working (almost) flawlessly.]
After much fiddling about with the firewall settings and turning screen sharing on and off I finally discovered a solution! Using the first answer from Greg Cairn on this site: http://apple.stackexchange.com/questions/43765/restart-screen-sharing-service-on-mac-mini-server/ .
My symptoms were that the “Share Screen…” button would show in the finder window on the Leopard machine, but clicking on this would immediately pop up a dialogue asking me to check that Screen Sharing was running on the target machine and the network was working ok. They both were.
Checking the app firewall.log in Console showed that the requests were being accepted and allowed through the firewall. So it appeared that the screensharingd itself wasn’t working.
Typing the following in a Terminal window su’d to a user with administrator privileges seems to restart screen sharing
sudo launchctl unload /System/Library/LaunchDaemons/com.apple.screensharing.plist sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.screensharing.plist
I was then asked by the Firewall to allow screensharingd to accept incoming network connections, which I did and it all started working again. 🙂