"Broken pipe" between Nginx and PHP-FPM

On 6th January, I updated my Drupal 8 development system (composer update). The following ls listing shows the modules that were updated.

drwxr-xr-x  7 drupal8 www-data 4096 Jan  6 02:36 checklistapi
drwxr-xr-x  7 drupal8 www-data 4096 Jan  6 02:36 ctools
drwxr-xr-x  4 drupal8 www-data 4096 Jan  6 02:36 quick_node_clone
drwxr-xr-x  6 drupal8 www-data 4096 Jan  6 02:36 workbench_access
drwxr-xr-x  4 drupal8 www-data 4096 Jan  6 02:36 file_checker
drwxr-xr-x  6 drupal8 www-data 4096 Jan  6 02:36 eva
drwxr-xr-x  4 drupal8 www-data 4096 Jan  6 02:36 social_post_linkedin
drwxr-xr-x  5 drupal8 www-data 4096 Jan  6 02:36 term_csv_tree_import
drwxr-xr-x  5 drupal8 www-data 4096 Jan  6 02:35 memcache
drwxr-xr-x  6 drupal8 www-data 4096 Jan  6 02:35 feeds
drwxr-xr-x  5 drupal8 www-data 4096 Jan  6 02:35 easy_breadcrumb
drwxr-xr-x  7 drupal8 www-data 4096 Jan  6 02:35 fontawesome
drwxr-xr-x  3 drupal8 www-data 4096 Jan  6 02:34 term_csv_export_import
drwxr-xr-x  4 drupal8 www-data 4096 Jan  6 02:34 leaflet_more_maps
drwxr-xr-x 11 drupal8 www-data 4096 Jan  6 02:34 devel
drwxr-xr-x  4 drupal8 www-data 4096 Jan  6 02:34 typed_data
drwxr-xr-x  5 drupal8 www-data 4096 Jan  6 02:34 externalauth
drwxr-xr-x  5 drupal8 www-data 4096 Jan  6 02:34 libraries
drwxr-xr-x  4 drupal8 www-data 4096 Jan  6 02:34 social_post_twitter
drwxr-xr-x  4 drupal8 www-data 4096 Jan  6 02:34 auto_entitylabel
drwxr-xr-x  5 drupal8 www-data 4096 Jan  6 02:33 contentimport
drwxr-xr-x 11 drupal8 www-data 4096 Jan  6 02:33 paragraphs
drwxr-xr-x  5 drupal8 www-data 4096 Jan  6 02:33 telephone_validation
drwxr-xr-x  9 drupal8 www-data 4096 Jan  6 02:32 leaflet
drwxr-xr-x  9 drupal8 www-data 4096 Jan  6 02:32 entity_browser
drwxr-xr-x  5 drupal8 www-data 4096 Jan  6 02:32 workflow
drwxr-xr-x  5 drupal8 www-data 4096 Jan  6 02:32 config_update
drwxr-xr-x  5 drupal8 www-data 4096 Jan  6 02:32 entity
drwxr-xr-x  6 drupal8 www-data 4096 Jan  6 02:32 workbench
drwxr-xr-x  3 drupal8 www-data 4096 Jan  6 02:32 unique_field
drwxr-xr-x  3 drupal8 www-data 4096 Jan  6 02:32 entity_reference_display
drwxr-xr-x  6 drupal8 www-data 4096 Jan  6 02:31 ledger
drwxr-xr-x  8 drupal8 www-data 4096 Jan  6 02:31 eu_cookie_compliance
drwxr-xr-x  3 drupal8 www-data 4096 Jan  6 02:31 social_api
drwxr-xr-x  3 drupal8 www-data 4096 Jan  6 02:31 permissions_filter
drwxr-xr-x  3 drupal8 www-data 4096 Jan  6 02:30 social_post
drwxr-xr-x  6 drupal8 www-data 4096 Jan  6 02:30 gdpr_compliance

I then went to bed and carried on working at around 14:20. Since that time I have regular errors of the following kind in my php7.2-fpm.log:

[06-Jan-2019 14:37:21] WARNING: [pool www] child 1263 said into stderr: "There was a problem sending 4325 bytes on socket 5: Broken pipe"
[06-Jan-2019 14:37:22] WARNING: [pool www] child 1263 said into stderr: "There was a problem sending 178 bytes on socket 5: Broken pipe"
[06-Jan-2019 14:40:23] WARNING: [pool www] child 1232 said into stderr: "There was a problem sending 178 bytes on socket 5: Broken pipe"
[06-Jan-2019 14:43:55] WARNING: [pool www] child 6461 said into stderr: "There was a problem sending 178 bytes on socket 5: Broken pipe"

I have only just noticed this now whilst in pursuit of another problem. Does anyone know what might be causing it?

I’m running Drupal 8.6 on Ubuntu 18.04 with Nginx 1.14.0 and PHP 7.2.10 with FPM. The database is Postgresql 10.6.

I thought originally it might be a hardware problem, but the the immediacy of the problem after the Drupal update seems too obvious to ignore.