I had the same problem and I found the following error was happening in wp-includes/js/tinymce/tiny_mce_gzip.php:
“Allowed memory size of 8388608 bytes exhausted (tried to allocate xxxxx bytes)”
So I changed this file, adding the line
ini_set('memory_limit','16M');
after the line
@require_once('../../../wp-config.php');
and this solved the problem.
Hope this can help.
