Unknown toolbar item "DrupalBreak"

Did you comment out the following line in fckeditor.config.js?

FCKConfig.Plugins.Add( 'drupalbreak' ) ; 

it should be uncommented if you want to use DrupalBreak button.

Alternatively, you may remove it from toolbar (FCKConfig.ToolbarSets).

Remember to clear browser's cache (and any other cache) after making changes in the javascript file.

I had the same problem when I upgraded to 6.x-1.3-rc5. I finally figured out that somehow I'd lost my plugins folder in the upgrade. Check to see if you still have the modules/fckeditor/plugins directory with drupalbreak, drupalpagebreak, and imgassist in it.