The error was detected when smushing S3 images manually in media library. While investigating the Wordpress debug log the error could be pinpointed to the entry:
2018-06-14T10:32:17+00:00 CRITICAL Uncaught Error: Call to undefined method DeliciousBrains\WP_Offload_S3\Providers\AWS_Provider::doesObjectExist() in /********/plugins/wp-smush-pro/lib/class-wp-smush-s3.php:352
It seems that the function is called before the WP offload S3 plugin has fully loaded and initialised. Although the smushed images work this error effectively cripples the media library (e.g no further images can be loaded past the erroneous call while browsing the library and posts using the smushed images won't be able to load or select images in the post editor because of the error also breaking JS).
Fast responses and solutions are much appreciated.