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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 18.0.2): 19
Operating system and version (eg, Ubuntu 20.04): CentOS 7
Apache or nginx version (eg, Apache 2.4.25): Nginx 1.17.10
PHP version (eg, 7.1): 7.4.7
The issue you are facing: Could not connect to appstore. Curl error 28. Out of -1 bytes received.
How’s -1 possible?
Is this the first time you’ve seen this error? (Y/N): N It’s been in the log many times.
Steps to replicate it:
The output of your Nextcloud log in Admin > Logging:
{"reqId":"pTdj88VgvAtT1H8","level":2,"time":"2020-07-02T23:05:18+00:00","remoteAddr":"9.8.7.6","user":"admin","app":"appstoreFetcher","method":"GET","url":"/settings/apps","message":"Could not connect to appstore: cURL error 28: Operation timed out after 10000 milliseconds with 1384458 out of -1 bytes received (see https://curl.haxx.se/libcurl/c/libcurl-errors.html)","userAgent":"Mozilla/5.0 (Linux; Android 10; SM-G975F) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.116 Mobile Safari/537.36","version":"19.0.0.12"}
{"reqId":"zOSFF1zcp1zGBlA","level":2,"time":"2020-07-03T13:00:15+00:00","remoteAddr":"","user":"--","app":"appstoreFetcher","method":"","url":"--","message":"Could not connect to appstore: cURL error 28: Connection timed out after 10001 milliseconds (see https://curl.haxx.se/libcurl/c/libcurl-errors.html)","userAgent":"--","version":"19.0.0.12"}
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => 'oc17.........hj',
'passwordsalt' => '...............',
'secret' =>'........ ',
'trusted_domains' =>
array (
0 => 'mydomain.com',
),
'datadirectory' => '/home/nginx/nextcloud/data',
'log_rotate_size' => 2097152,
'dbtype' => 'mysql',
'version' => '19.0.0.12',
'overwrite.cli.url' => 'https://mydomain.com',
'enable_previews' => true,
'enabledPreviewProviders' =>
array (
0 => 'OC\\Preview\\PNG',
1 => 'OC\\Preview\\JPEG',
2 => 'OC\\Preview\\GIF',
3 => 'OC\\Preview\\BMP',
4 => 'OC\\Preview\\XBitmap',
5 => 'OC\\Preview\\Movie',
6 => 'OC\\Preview\\PDF',
7 => 'OC\\Preview\\MP3',
8 => 'OC\\Preview\\TXT',
9 => 'OC\\Preview\\MarkDown',
),
'preview_max_x' => 1024,
'preview_max_y' => 1024,
'memcache.local' => '\\OC\\Memcache\\Redis',
'filelocking.enabled' => 'true',
'memcache.locking' => '\\OC\\Memcache\\Redis',
'overwriteprotocol' => 'https',
'redis' =>
array (
'host' => '/var/run/redis/redis.sock',
'port' => 0,
'timeout' => 0.0,
),
'dbname' => 'dbdata',
'dbhost' => 'localhost:3306',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => 'nextcloud',
'dbpassword' => 'abcqwer',
'installed' => true,
'mail_from_address' => 'nimda',
'mail_smtpmode' => 'smtp',
'mail_sendmailmode' => 'smtp',
'mail_domain' => 'mydomain.com',
'maintenance' => false,
'updater.release.channel' => 'beta',
'theme' => '',
'loglevel' => 2,
);
The output of your Apache/nginx/system log in /var/log/____:
PASTE HERE
I can connect with curl.
curl -Iv https://apps.nextcloud.com * About to connect() to apps.nextcloud.com port 443 (#0) * Trying 88.198.160.138...
* Connected to apps.nextcloud.com (88.198.160.138) port 443 (#0)
* Initializing NSS with certpath: sql:/etc/pki/nssdb
* CAfile: /etc/pki/tls/certs/ca-bundle.crt
CApath: none
* SSL connection using TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
* Server certificate:
* subject: CN=apps.nextcloud.com
* start date: May 19 01:02:18 2020 GMT
* expire date: Aug 17 01:02:18 2020 GMT
* common name: apps.nextcloud.com
* issuer: CN=Let's Encrypt Authority X3,O=Let's Encrypt,C=US
> HEAD / HTTP/1.1
> User-Agent: curl/7.29.0
> Host: apps.nextcloud.com
> Accept: */*
>
< HTTP/1.1 200 OK
HTTP/1.1 200 OK
< Date: Fri, 03 Jul 2020 15:57:42 GMT
Date: Fri, 03 Jul 2020 15:57:42 GMT
< Server: Apache/2.4.18 (Ubuntu)
Server: Apache/2.4.18 (Ubuntu)
< Strict-Transport-Security: max-age=15768000; includeSubDomains; preload
Strict-Transport-Security: max-age=15768000; includeSubDomains; preload
< Vary: Accept-Language,Cookie,Accept-Encoding
Vary: Accept-Language,Cookie,Accept-Encoding
< X-XSS-Protection: 1; mode=block
X-XSS-Protection: 1; mode=block
< X-Frame-Options: SAMEORIGIN
X-Frame-Options: SAMEORIGIN
< Content-Security-Policy: default-src 'none'; style-src 'self'; form-action 'self'; img-src *; script-src 'self'; font-src 'self'; connect-src 'self'
Content-Security-Policy: default-src 'none'; style-src 'self'; form-action 'self'; img-src *; script-src 'self'; font-src 'self'; connect-src 'self'
< Content-Language: en
Content-Language: en
< X-Content-Type-Options: nosniff
X-Content-Type-Options: nosniff
< Content-Length: 178177
Content-Length: 178177
< Content-Type: text/html; charset=utf-8
Content-Type: text/html; charset=utf-8
<
* Connection #0 to host apps.nextcloud.com left intact
This question has already been asked and answered several times, as you can see here: