After activating the plugin Version: 126.96.36.199, all of the existing images (.jpg) in the media gallery are gone, just the file name appears.
The links to the images are still intact in the wp-content folder.
(I did not do a "bulk smush it", I simply activated the plugin. After deactivating it, the image thumbnails are still not showing.
I found something about resetting the file permissions to default but I don't know how to do this.