After an upgrade from WP core version 4.8.3 to version 4.9.1, the media library does not display anything except the upload button … unable to upload.
Editing a post or page, are able to view the attached featured image.
“upload” directory permissions are correct (maintained by automated deployment processes.)
Disabled all plugins, no change, no joy.
Downgraded to core version 4.9, no change.
No java script errors, no web server log errors, the mystery deepens.
Installed current version of the twentyseventeen theme, no change, problem still exists.
The surface appearance is that the core 4.9 contains a bug that affects some themes, potentially an issue with the installed theme.
Apache web server environment variable used by a custom mu-plugin.
This was missing from the web server configuration.
In theory, the custom mu-plugin provides a default value if the environment variable is missing … until core version 4.9.
Just guessing, better security related handling of “stuff” in version 4.9 correctly broke the mu-plugin … now fixed, problem resolved.
