In my case, Environment.NewLine was working fine while previewing the report in Visual Studio. But when I tried to publish the rdl to Dynamics 365 CE, I received the error "The report server has RDLSandboxing enabled and the Value expression for the text box 'Textbox10' contains a reference to a type, namespace, or member 'Environment' that is not allowed."
So I had to replace Environment.NewLine with vbcrlf.