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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, 20.0.5): 24.0.4
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04 with Docker
PHP version (eg, 7.4): 8.0
The issue you are facing:
Is this the first time you’ve seen this error? (Y/N):
Couldn’t backup for a couple of months, now I have the time I wanna dive deeper into the issue. I got new info on it since today.
Steps to replicate it:
- Install backup app
- Try to set app data folder
- Cannot click on the button
Within shell:
- Login as www-data
- Do the following command within the NC folder:
php occ backup:external:appdata. Even with a specific external storage number it throws the same error.
The output of your Nextcloud log in Admin > Logging:
In ExternalFolderService.php line 354:
[OCA\Backup\Exceptions\ExternalFolderNotFoundException]
backup:external:appdata [--unset]
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
$CONFIG = array (
'htaccess.RewriteBase' => '/',
'memcache.local' => '\\OC\\Memcache\\APCu',
'apps_paths' =>
array (
0 =>
array (
'path' => '/var/www/html/apps',
'url' => '/apps',
'writable' => false,
),
1 =>
array (
'path' => '/var/www/html/custom_apps',
'url' => '/custom_apps',
'writable' => true,
),
),
'instanceid' => 'RECACTED',
'passwordsalt' => 'RECACTED',
'secret' => 'RECACTED',
'trusted_domains' =>
array (
0 => 'RECACTED',
1 => 'RECACTED',
2 => 'RECACTED',
3 => 'RECACTED',
),
'datadirectory' => '/var/www/html/data',
'dbtype' => 'mysql',
'version' => '24.0.4.1',
'overwrite.cli.url' => 'RECACTED',
'overwriteprotocol' => 'https',
'dbname' => 'nextcloud',
'dbhost' => 'nextcloud-mariadb',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => 'nextcloud',
'dbpassword' => 'RECACTED',
'installed' => true,
'default_phone_region' => 'RECACTED',
'onlyoffice' =>
array (
'DocumentServerInternalUrl' => 'RECACTED',
'StorageUrl' => 'RECACTED',
'DocumentServerUrl' => '/ds-vpath/',
),
'allow_local_remote_servers' => 'false',
'maintenance' => false,
'loglevel' => 2,
'app_install_overwrite' =>
array (
0 => 'metadata',
1 => 'files_external_gdrive',
2 => 'bruteforcesettings',
3 => 'drawio',
4 => 'extract',
5 => 'ncdownloader',
6 => 'uploaddetails',
),
'trashbin_retention_obligation' => '30, 60',
'versions_retention_obligation' => '120, 240',
'updater.release.channel' => 'stable',
'mail_from_address' => 'RECACTED',
'mail_smtpmode' => 'smtp',
'mail_sendmailmode' => 'smtp',
'mail_domain' => 'RECACTED',
'mail_smtpsecure' => 'tls',
'mail_smtphost' => 'RECACTED',
'mail_smtpauthtype' => 'LOGIN',
'mail_smtpauth' => 1,
'mail_smtpport' => '587',
'mail_smtpname' => 'RECACTED',
'mail_smtppassword' => 'RECACTED',
'theme' => '',
);