If WordPress’ visual editor TinyMCE stops working after you have edited the config file tiny_mce_config.php and begins droping error messages telling you “Cannot modify header information – headers already sent”, make sure the config file doesn’t start with a byte-order mark (e. g. 0xEF 0xBB 0xBF). It seems Windows Notepad writes these bytes to the file if you edit and save it with Notepad.

To get TinyMCE running again, get a fresh copy of the config file and overwrite your flawed one with it. If you need to edit the config file try a different editor. WordPad did the job for me. Just to make sure you are not running into the same problem again, check the edited file with a hex editor of your choice and see that it’s not starting with EF BB BF.