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MacFixIt> reports others with 10.4.11 having problems, also. Their edited suggestions for correcting:
1 Reinstall QuickTime (not clear if this is 7.6 or an older version, you
do have the archived original, don't you?)
2 Remove ~/Library/com.apple.quicktime.plugin.preferences.plist
3 Remove QuickTime Plugin.plugin (from /Library/Internet Plugins)
4 search for conflicting, third-party plugins and remove them
Take a look at the comments, also, seems the ...plugin.preferences.plist solved most peoples problems.