js error on post editing page

I’d suggest disabling script concatenation and/or compression to see if that helps. You can do this by adding the following to the wp-config.php file:

define( 'CONCATENATE_SCRIPTS', false );
define( 'COMPRESS_SCRIPTS', false );

And maybe even script debugging to..

define( 'SCRIPT_DEBUG', true );

Are you using any caching plugins?

EDIT:
The most common reason for seeing that message seems to be running an older version of jQuery, ie. anything lower than 1.4.2 … do you have any plugins that load jQuery?