cURL error 60: SSL certificate problem

Server
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I recently started receiving this message in my ownCloud log after viewing the Apps page:

Error - core - Could not get application: cURL error 60: SSL certificate problem: unable to get local issuer certificate

It was working earlier last night, and I did not make any changes to the internal configuration of my server, or my ownCloud configuration. I was however making some changes to the example theme, and uploading them to another theme folder in /themes/. However, I don't see how that could really cause any issue with this.

I've been searching around a lot regarding how to fix this issue. Here's some information, and what I have tried:
ownCloud version: 9.1.2.5 (stable)
Server version: Ubuntu 16.04.1 LTS

I previously was not able to get an output from curl -i https://apps.owncloud.com because of the certificate not being in my bundle. I used:

echo -n | openssl s_client -showcerts -connect apps.owncloud.com:443 2>/dev/null  | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p'

to get the certificate of that page, and then I added it into my /etc/ssl/certs/ca-certificates.crt file. After doing that, I was able to perform a successful call on curl -i https://apps.owncloud.com. However, my issue was not resolved, the Apps page still does not load all of the categories (Multimedia, Games, Tools, etc.), and the error appears in the log still.

In my config.php, I have the following (I removed some items that wouldn't be related):

<?php
$CONFIG = array (
  'updatechecker' => false,
  'datadirectory' => '/var/www/owncloud/data',
  'version' => '9.1.2.5',
  'logtimezone' => 'America/New_York',
  'installed' => true,
  'appstoreenabled' => true,
  'appstore.experimental.enabled' => true,
  'maintenance' => false,
);

My servers SSL certificate is provided by Lets Encrypt, and does validate fine (checked on SSL Labs). Some solutions I found online mentioned downloading a ca-bundles.crt that needed to go into my config folder, but it wasn't mentioned in the 9.1.x setup documentation, so I assume that it's not related. Any help would be appreciated. For now I have disabled the appstore via the config file.

2

Known appstore issue

3

So closing as a duplicate.