If you've noticed, the FCKEditor doesn't work too well with the RequiredFieldValidator. The RequiredFieldValidator reports invalid data indicating that no value has been provided for the FCKEditor. However, when you cause a postback again to submit the value, all is well.
This can be pretty annoying since you would have to write a custom validator, but for a quick fix you can just disable client-side validation on the RequiredFieldValidator by setting the EnableClientScript to false. Make sure the submit button or whatever control you are using has a handler that checks for Page.IsValid before you do anything... when you disable the client-side code, all the server-side code for the validators do is set the IsValid property of the page are let your event handlers execute normally.