It seems that your richtextbox control is generating HTML code. It tries to pass HTML value into server. By default, this is not allowed. To allow it, you need to configure your Web.config:
this is wrong:
<system.web>
<httpruntime requestvalidationmode="2.0" />
</system.web>
this is correct:
<system.web>
<httpruntime requestValidationMode="2.0" />
</system.web>