Error with ASP.NET 2.0

Hello!

i want to use fckeditor in my asp.net application but i allways getting a strange error:
[Void ValidateString(System.String, System.String, System.String) at line 0]
Ein möglicherweise gefährlicher Request.Form-Wert wurde vom Client (ctl00$ContentPlaceHolder1$input_detailsde="<p>teste</p>") entdeckt.
bei System.Web.HttpRequest.ValidateString(String s, String valueName, String collectionName)
bei System.Web.HttpRequest.ValidateNameValueCollection(NameValueCollection nvc, String collectionName)
bei System.Web.HttpRequest.get_Form()
bei System.Web.HttpRequest.get_HasForm()
bei System.Web.UI.Page.GetCollectionBasedOnMethod(Boolean dontReturnNull)
bei System.Web.UI.Page.DeterminePostBackMode()
bei System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

i have a masterpage und severale pages which uses this masterpage. on one of them i want to use the editor. it is shown normally und everything. but when i try to save the value to my database or refresh the page, there occurs this error (translated in english it says: "a possibly unsafe request.form-value from client ... has been discoverd").

any ideas! thx in advance, h4nni