This error is returned only by the setter of the Text server property of RadEditor and is available only in this property in the whole source code of the control:
InvalidOperationException(@"Please, use the Content property of RadEditor to
The value of the Text property
the value of the Content property.");
We have not made any changes inside this functionality and you should not experience the reported problem with the latest version 2011.1.517. Could you please isolate the problem in a simple working project and send it for examination on our end via the attachment functionality of the support ticketing system.
the Telerik team