[Bug]: External SFTP storage randomly stops working / login failing

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• Nextcloud Server is running on 64bit capable CPU, PHP and OS.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

I'm trying to connect an external SFTP storage (SSH on Ubuntu 22.04) to my Nextcloud (for backup reasons...). No matter what server or hostname I use, The indicator shows green (~10%) and red (~90%) randomly whenever I test the connection.
I was able to connect to all SFTP via ssh or FileZilla for months without issues so I know they work well and respond fast. I also tried localhost and 127.0.0.1 which never worked.

10% of the time:

90% of the time:

This bug seems to be similar to #31851 (Apr 5, 2022: External SFTP storage randomly stops working), which was closed due to inactivity.

Steps to reproduce

1. Enter SFTP connection details and credentials into the form within the external storage app.
2. Enter tested and valid SFTP connections.
3. Reload the indecator.

Expected behavior

The indicator shows green (~10%) and red (~90%) randomly whenever I test the connection.
I cannot open the external storage under files: The folder gets a red background after some time when clicked.

Installation method

Community Web installer on a VPS or web space

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.1

Web server

Apache (supported)

Database engine version

MariaDB

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "***REMOVED SENSITIVE VALUE***"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "25.0.4.1",
        "overwrite.cli.url": "***REMOVED SENSITIVE VALUE***",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "default_phone_region": "DE",
        "default_language": "de_DE",
        "default_locale": "de",
        "memcache.local": "\\OC\\Memcache\\APCu",
        "skeletondirectory": "",
        "maintenance": false
    }
}

List of activated Apps

Enabled:
  - activity: 2.17.0
  - backup: 1.2.0
  - circles: 25.0.0
  - cloud_federation_api: 1.8.0
  - cms_pico: 1.0.21
  - collectives: 2.3.0
  - comments: 1.15.0
  - contacts: 5.1.0
  - contactsinteraction: 1.6.0
  - dashboard: 7.5.0
  - dav: 1.24.0
  - diary: 0.0.9
  - federatedfilesharing: 1.15.0
  - federation: 1.15.0
  - files: 1.20.1
  - files_external: 1.17.0
  - files_pdfviewer: 2.6.0
  - files_rightclick: 1.4.0
  - files_sharing: 1.17.0
  - files_trashbin: 1.15.0
  - files_versions: 1.18.0
  - firstrunwizard: 2.14.0
  - groupfolders: 13.1.1
  - logreader: 2.10.0
  - lookup_server_connector: 1.13.0
  - nextcloud_announcements: 1.14.0
  - notifications: 2.13.1
  - oauth2: 1.13.0
  - password_policy: 1.15.0
  - photos: 2.0.1
  - privacy: 1.9.0
  - provisioning_api: 1.15.0
  - recommendations: 1.4.0
  - related_resources: 1.0.4
  - richdocuments: 7.1.2
  - richdocumentscode: 22.5.802
  - serverinfo: 1.15.0
  - settings: 1.7.0
  - sharebymail: 1.15.0
  - spreed: 15.0.4
  - support: 1.8.0
  - survey_client: 1.13.0
  - systemtags: 1.15.0
  - text: 3.6.0
  - theming: 2.0.1
  - twofactor_backupcodes: 1.14.0
  - updatenotification: 1.15.0
  - user_status: 1.5.0
  - viewer: 1.9.0
  - weather_status: 1.5.0
  - workflowengine: 2.7.0
Disabled:
  - admin_audit
  - bruteforcesettings
  - encryption
  - suspicious_login
  - twofactor_totp
  - user_ldap

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

This error many times:

files_external	Exception: Login failed
/var/www/ncTest/apps/files_external/lib/Lib/Storage/SFTP.php - line 166:
OCA\Files_External\Lib\Storage\SFTP->getConnection()
/var/www/ncTest/apps/files_external/lib/MountConfig.php - line 131:
OCA\Files_External\Lib\Storage\SFTP->test()
/var/www/ncTest/apps/files_external/lib/Controller/StoragesController.php - line 288:
OCA\Files_External\MountConfig::getBackendStatus("*** sensiti ... *")
/var/www/ncTest/apps/files_external/lib/Controller/GlobalStoragesController.php - line 201:
OCA\Files_External\Controller\StoragesController->updateStorageStatus("*** sensiti ... *")
/var/www/ncTest/lib/private/AppFramework/Http/Dispatcher.php - line 225:
OCA\Files_External\Controller\GlobalStoragesController->update("*** sensiti ... *")
/var/www/ncTest/lib/private/AppFramework/Http/Dispatcher.php - line 133:
OC\AppFramework\Http\Dispatcher->executeController()
/var/www/ncTest/lib/private/AppFramework/App.php - line 172:
OC\AppFramework\Http\Dispatcher->dispatch()
/var/www/ncTest/lib/private/Route/Router.php - line 298:
OC\AppFramework\App::main()
/var/www/ncTest/lib/base.php - line 1047:
OC\Route\Router->match()
/var/www/ncTest/index.php - line 36:
OC::handleRequest()

Additional info

The server shows a low CPU and RAM usage throughout testing. php.ini was configured to have more than enough resources available:

max_execution_time = 10800 
memory_limit = 512M 
post_max_size = 10G 
upload_max_filesize = 10G 
date.timezone = Europe/Berlin 
opcache.enable=1 
opcache.memory_consumption=512 
opcache.interned_strings_buffer=64 
opcache.max_accelerated_files=10000 
opcache.revalidate_freq=1 
opcache.save_comments=1 

Start with apps/files_external/lib/Storage/SFTP.php at line 166 and trace the logged call through MountConfig.php and the storage controllers. Repeatedly test the valid SFTP connection described in the issue, then verify that the connection indicator and external folder remain reliable rather than failing intermittently.