I did not intend that behaviour.
I have completed step #1 of the debugging doc. All non Generate Press plugins are disabled.
It is still not loading an archive, but the message has changed to
This XML file does not appear to have any style information associated with it. The document tree is shown below.
<![CDATA[ WordPress › Error ]]>
<![CDATA[ <strong>Error</strong>: This is not a valid feed template. ]]>