I found this thread: https://premium.wpmudev.org/forums/topic/wp-smush-pro-disabling-media-library while search for answers.
I have a multisite install. On the primary site (bethnicol.com) when WP Smush Pro is activated the media library is broken -- but not completely. If I use the list view, I can see all of my images, but many of them have an error ; These do not show up at all when I use the "thumbnail view" -- All is happy when I deactivate the plugin.
I am on the latest version of the plugin (2.1.2); Images that I used in posts prior to the plugin installation are displaying fine in posts.