This error is encountered when the content has HTML tags in it. 

Please follow the steps below to get rid of this error.

1.      Open you web.config file and add the entry of <httpRuntime requestValidationMode="2.0" />  under the <System.web>

<system.web>
            .....

    <httpRuntime requestValidationMode="2.0" />

</system.web>

2.      Now open the page on which you are getting this error and add ValidateRequest="false" in the page directive.

<%@Page Language="C#" AutoEventWireup="true" ValidateRequest="false" %>

Referenced Link >  Troubleshooting: A potentially dangerous Request.Form value was detected 

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