smilinghuman

I was having the same exact problem and then I found the following post: http://ww.wp.xz.cn/support/topic/156276.
Scroll down to the reply by vizthink and you will see a solution. I also installed the Advanced editor and this solution did not work for me until I uninstalled it. Once I removed the editor, all was good. Now maybe you could look into the editor and find a similar location to change. I did not really need the advanced features, so I just uninstalled it.

Pertinent details from that reply with the implied “at your own risk”

1. Locate the file ‘tiny_mce_config.php’ ( it should live in %your_blog_root%/wp-includes/js/tinymce/ )
2. Assuming you want to avoid catastrophe if things go awry, back this file up!
3. Open this file in any text editor and locate the following code ( Line 298 for me )

   // Add external plugins and init
   $content .= $ext_plugins . 'tinyMCE.init({' . $mce_options . '});';

4. Replace with this ( or a variation to suit your own needs )

   // Add external plugins and init
   $content .= $ext_plugins . 'tinyMCE.init({extended_valid_elements : "iframe[id|class|title|style|align|frameborder|height|longdesc|marginheight|marginwidth|name|scrolling|src|width]",' . $mce_options . '});';

5. Save the file back to its original location
6. Clear your browser cache ( for good measure )
7. While technically unnecessary, you can also delete or rename the folder ‘js_cache’ ( it should live in %your_blog_root%/wp-content/uploads/ )
8. Test your new settings with the desired code by toggling back and forth between the ‘Visual’ and ‘HTML’ tabs at least once. Return back to the HTML view to verify the code is still there and finally back to the ‘Visual’ tab to save/publish a test post.