Found this issue on a client site too.
The admin-ajax.php throws a 500 error when viewing the media library.
Emailed diagnostic info to nom@ address.
Reverted back to 1.4.3 and everything is back to normal!
Here’s the fatal error – looks to be an incompatibility with Smush?
[03-Oct-2018 22:00:19 UTC] PHP Fatal error: Call to undefined method Amazon_S3_And_CloudFront::is_attachment_served_by_s3() in /home/username/public_html/wp-content/plugins/wp-smushit/lib/integrations/class-wp-smush-s3.php on line 265
[03-Oct-2018 22:01:19 UTC] PHP Fatal error: Call to undefined method Amazon_S3_And_CloudFront::is_attachment_served_by_s3() in /home/username/public_html/wp-content/plugins/wp-smushit/lib/integrations/class-wp-smush-s3.php on line 265
[03-Oct-2018 22:01:21 UTC] PHP Fatal error: Call to undefined method Amazon_S3_And_CloudFront::is_attachment_served_by_s3() in /home/username/public_html/wp-content/plugins/wp-smushit/lib/integrations/class-wp-smush-s3.php on line 265
[03-Oct-2018 22:03:56 UTC] PHP Fatal error: Call to undefined method Amazon_S3_And_CloudFront::is_attachment_served_by_s3() in /home/username/public_html/wp-content/plugins/wp-smushit/lib/integrations/class-wp-smush-s3.php on line 265