React 16: Warning: Expected server HTML to contain a matching <div> in <body> : <p>Since upgrading to React 16 I get this error message:</p>

<p><code>warning.js:33 Warning: Expected server HTML to contain a matching &lt;div&gt; in &lt;body&gt;.</code></p>

<p>What is generally causing this error message and how can it be fixed?</p>
