PG::UnableToSend: server closed the connection unexpectedly

Very stock cloud install.
Here’s discourse-doctor’s output.

DISCOURSE DOCTOR Fri Jun 28 02:00:11 UTC 2019
OS: Linux community.example.org 4.4.0-151-generic #178-Ubuntu SMP Tue Jun 11 08:30:22 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux


Found containers/app.yml

==================== YML SETTINGS ====================
DISCOURSE_HOSTNAME=community.example.org
SMTP_ADDRESS=smtp.mailgun.org
DEVELOPER_EMAILS=REDACTED 
SMTP_PASSWORD=REDACTED 
SMTP_PORT=587
SMTP_USER_NAME=postmaster@community.example.org
LETSENCRYPT_ACCOUNT_EMAIL=REDACTED 

==================== DOCKER INFO ====================
DOCKER VERSION: Docker version 18.09.6, build 481bc77

DOCKER PROCESSES (docker ps -a)

CONTAINER ID        IMAGE                           COMMAND             CREATED             STATUS              PORTS                                      NAMES
c8d61fb69150        local_discourse/app             "/sbin/boot"        2 months ago        Up About a minute   0.0.0.0:80->80/tcp, 0.0.0.0:443->443/tcp   app
612da87d4e61        local_discourse/mail-receiver   "/sbin/boot"        14 months ago       Up About a minute   0.0.0.0:25->25/tcp                         mail-receiver


Discourse container app is running


==================== PLUGINS ====================
          - git clone https://github.com/discourse/docker_manager.git
          - git clone https://github.com/angusmcleod/discourse-legal-tools.git
          - git clone https://github.com/angusmcleod/discourse-custom-wizard.git
          - git clone https://github.com/discourse/discourse-solved.git

WARNING:
You have what appear to be non-official plugins.
If you are having trouble, you should disable them and try rebuilding again.

See https://github.com/discourse/discourse/blob/master/lib/plugin/metadata.rb for the official list.

========================================
Discourse version at community.example.org: Discourse 2.4.0.beta1 
Discourse version at localhost: Discourse 2.4.0.beta1 


==================== MEMORY INFORMATION ====================
OS: Linux
RAM (MB): 4046

              total        used        free      shared  buff/cache   available
Mem:           3951        1226        1359          64        1364        2356
Swap:             0           0           0

==================== DISK SPACE CHECK ====================
---------- OS Disk Space ----------
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1        60G   14G   43G  24% /

---------- Container Disk Space ----------
Filesystem      Size  Used Avail Use% Mounted on
overlay          60G   14G   43G  24% /
/dev/vda1        60G   14G   43G  24% /shared
/dev/vda1        60G   14G   43G  24% /var/log

==================== DISK INFORMATION ====================
Disk /dev/vda: 60 GiB, 64424509440 bytes, 125829120 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xc4c394c3

Device     Boot Start       End   Sectors Size Id Type
/dev/vda1  *     2048 125829086 125827039  60G 83 Linux

==================== END DISK INFORMATION ====================

==================== MAIL TEST ====================
For a robust test, get an address from http://www.mail-tester.com/
Mail test skipped.

==================== DONE! ====================