Hi ludwig,

If you are testing under .net framework 4.x, please try the setting below in your web.config. 

<httpRuntime requestValidationMode=″2.0″ />

Also, you can try set property "EncodeHiddenValue" to true.

<RTE:Editor ID="editor1" runat="server"  EncodeHiddenValue="true"/> 

Regards,

Ken