A potentially dangerous Request.Form value was detected from the client (ctl00$PlaceHolderMain$...

Recently I got the following errormessage in SharePoint 2013 after trying to save an edited publishing page. Also it was not possible to add any webpart.
Some googling resultet in different web.config-modifications, some said to set requestValidationMode="4.0" to requestValidationMode="2.0" in <system.web>-node, others recommended to set validateRequest="false"in <pages>-node. In fact for it only worked after I did both changes. So search for

<system.web>
 <httpRuntime requestValidationMode="4.0" />
</system.web>

and set requestValidationMode to "2.0" and search for

<pages enableSessionState="false" enableViewState="true" enableViewStateMac="true" validateRequest="true" clientIDMode="AutoID" pageParserFilterType="Microsoft.SharePoint.ApplicationRuntime.SPPageParserFilter, Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" asyncTimeout="7">
      <namespaces>
        <remove namespace="System.Web.UI.WebControls.WebParts" />
      </namespaces>
      <tagMapping>
        <add tagType="System.Web.UI.WebControls.SqlDataSource, System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" mappedTagType="Microsoft.SharePoint.WebControls.SPSqlDataSource, Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
      </tagMapping>
      <controls>
        <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
        <add tagPrefix="spsswc" namespace="Microsoft.Office.Server.Search.WebControls" assembly="Microsoft.Office.Server.Search, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
      </controls>
    </pages>

and set validateRequest to "false".