Ive fixed it by following these guidelines:
For others coming across the same error with version 2.51,
Edit the following file: yoursite/wp-includes/js/tinymce/tiny_mce_config.php
First, comment out (by instering // before the code) while ( @ob_end_clean() ); at around line 13. It is immediately preceded by // Discard any buffers
Then, search for “compress” until you find ‘compress’ => true and change the “true” to “false.
If you have the same problem with version 2.5, you only need to worry about the compression setting.