Mobile preview in V.9 not showing

The previews were working before. I haven’t done any upgrades. Is this common? What would cause this sudden lack of viewing? It is happening when I edit in firefox or Chrome.
Help! It shows part of the page but not the content…just the background

Are you able to take a screenshot? And send a snapshot of your Concrete CMS site’s configuration? The site configuration is under

This problem occurs quite often.
Two identical environments - in one is completely absent (fresh installation).

oops my oversight, the environments are not the same - the problem is in php 8, version 7.4 is ok