A potentially dangerous Request.Form value was detected from the client

Add this to your web.config:

<httpRuntime requestValidationMode="2.0" />

Have you tried setting the htmlEncodeOutput property?

CKEDITOR.replace('editor1', {
    htmlEncodeOutput: true });

This should encode the output and you should be able to avoid setting the requestValidationMode.

Documentation for it is here: ckEditor documentation