Thanks, Graham.

I tried putting the content controls in a borderless table first. This didn't work and still produced the error.

Then I tried putting the content controls on separate lines and yes, this did work and allowed having paragraphs without the error, but unfortunately this is not an ideal result for the layout of the finished form.

Having a browse online I stumbled across the mention of ContentControl.MultiLine property

Would this work somehow to overcome my error code and allow multi-line with the RichText content control?