Flash CSSEditor

Check the permissions of the file or folder AND file on your server. If on of them is write protected (chmod 644) you will get an error message. You need to change file and folder permissions to 755 or 777 depending on the Apache configuration of your host.
Commonly chmod 777 allows everyone to write to your server including this application. But some servers throw in this case an error 500 (Internal Server Error) and you have to reduce the permissions to chmod 755.