It turns out that the 10.4 Script Editor can only open scripts saved using the “default” scripting system. The “default” scripting system is the one that is assigned to new documents – generally AppleScript.
NOTE: this problem also effects Script Debugger’s AppleScript Debugging OSA component. If, for some reason, you need to open scripts saved from Script Debugger with Debugging enabled in the Script Editor, you’ll have to use the same trick.
Apple is aware of the issue and working on a fix. However, its not clear when a fix will be released.