We have a problem with one of your plugins.
As we have a lot (!) of images in our WP database.
WpSmushPro allocates too much memory which causes a HTTP500 for all logged in users.
To get this into perspective: 1GB of RAM is still not sufficient.
I analyzed the problem and found the error in /wp-content/plugins/wp-smush-pro/lib/class-wp-smush-db.php in the smushed_count() function.
Here you load all attachments with metadata into RAM to send it to your servers (which I find not ok to be honest!).
$query = array( 'fields' => array( 'ids', 'post_mime_type' ), 'post_type' => 'attachment', 'post_status' => 'inherit', 'order' => 'ASC', 'posts_per_page' => - 1, 'meta_key' => 'wp-smpro-smush-data', 'no_found_rows' => true );
DB says that there are 52680 images.