Nextcloud instance is not working: Internal Server Error: The server encountered an internal error and was unable to complete your request

Symptoms

• Nextcloud instance stop working after migration.

• The following error shows up in web browser:

  external Server Error

  The server encountered an internal error and was unable to complete your request.
  Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
  More details can be found in the server log.

• The following error shows up in Plesk Panel when accessing Extensions > My Extensions > Nextcloud:

  An unhandled exception has been thrown: RedisException: Connection refused in /var/www/vhosts/example.com/lib/private/RedisFactory.php:137
  Stack trace:
  #0 /var/www/vhosts/example.com/lib/private/RedisFactory.php(137): Redis->pconnect()
  #1 /var/www/vhosts/example.com/lib/private/RedisFactory.php(178): OCRedisFactory->create()
  #2 /var/www/vhosts/example.com/lib/private/Memcache/Redis.php(66): OCRedisFactory->getInstance()
  #3 /var/www/vhosts/example.com/lib/private/Memcache/Redis.php(72): OCMemcacheRedis->getCache()
  #4 /var/www/vhosts/example.com/lib/private/App/InfoParser.php(58): OCMemcacheRedis->get()
  #5 /var/www/vhosts/example.com/lib/private/App/AppManager.php(732): OCAppInfoParser->parse()
  #6 /var/www/vhosts/example.com/lib/private/legacy/OC_App.php(434): OCAppAppManager->getAppInfo()
  #7 /var/www/vhosts/example.com/lib/private/AppFramework/App.php(71): OC_App::getAppInfo()
  #8 /var/www/vhosts/example.com/lib/private/legacy/OC_App.php(155): OCAppFrameworkApp::buildAppNamespace()
  #9 /var/www/vhosts/example.com/lib/private/AppFramework/Bootstrap/Coordinator.php(119): OC_App::registerAutoloading()
  #10 /var/www/vhosts/example.com/lib/private/AppFramework/Bootstrap/Coordinator.php(90): OCAppFrameworkBootstrapCoordinator->registerApps()

• Redis database parameters are configured in /var/www/vhosts/example.com/nextcloud/config/config.php:

  'redis' =>
  array (
  'host' => 'localhost',
  'port' => 6379,
  ),

Cause

Nextcloud is trying to connect to redis database that does not exist or was not migrated.

Resolution

Use Docker native utilities such as docker export and docker import to migrate the container from one host to another or redeploy the redis container and manually move files from volume mapping.

Note: This is considered an administrative task that should be performed by the server administrator