If Apple Remote Desktop (ARD) continues to lock up when you open it, and it is looking at the Task History of the last performed task, you may need to simply delete the preference file containing the task history for "today". Here's how...
1. Close/Force Quit ARD.
2. Click on the Finder in the Dock. Then click the Go menu, and choose Home.
Navigate to and open the following path from your user home directory (keep in mind the ~/ denotes the current user's home path):
~/Library/Application Support/Remote Desktop/Task History/
3. Locate the plist that has today's (or the most recent/offending) date in it. It will follow this naming convention, YEAR-MONTH-DATE.plist.
For example: August 25, 2009 would be 2009-08-25.plist.
4. Locate and delete the file.
Now you should be able to open ARD. If you just so happen to be viewing another task upon opening ARD, other then today, then either delete the date when that task was ran. Or if your history is not required, simply delete all the plists in the path mentioned above.
Continuous lockup? Simply disable Task History in ARD
If this continues to occur, then you may want to simply disable Task History until Apple addresses this by:
1. Open Apple Remote Desktop.
2. Click on the Remote Desktop menu, and choose Preferences.
3. Click Tasks, and change Limit history shown in sidebar to the last 1 tasks. (see below). Check Limit task history to 1 days.
4. Close the window, then close and reopen Apple Remote Desktop.