I have cloned the site on another server and the problem doesn't occur there. The PHP version and the Drupal version (7.89) is the same.
How can I debug what is going on here?
Edit: Additional info
As I mentioned in the comments, the aggregated CSS files are not viewable in the browser. However, If I change the filename to e.g. test.css, it then is viewable.
Each aggregate file has a corresponding .gz of the same name. When I delete this the .css file is then viewable via the browser. So it looks like the issue has something to with these .gz files.