The layout preview in Designer is broken when targeting LTS2021. When navigating to an existing layout, it seems to be loading but it never ends up displaying anything. Checking on the browser console, amongst other errors there is:
This is particularly suspicious because there is no nuxeo-elements.js in @nuxeo, the correct path is @nuxeo/nuxeo-elements/nuxeo-elements.js.
This seems to be a problem only on 4.5.7 (and after), on 4.5.5 everything works fine.