Redis issue with previewgenerator -> 503 server

EDIT

My redis-server seems to be having all kinds of problems this morning, so that appears to be the main root cause of the problem. But I think the bug report here is still valid. It seems like there is no way to change the redis settings for the previewgenerator app, or it does not honour the settings in config.php when they are updated. Perhaps there is something I am missing, but I think that needs looking into.

Original report:

My server suddently started returning 503s today. Disabling the previewgenerator app solved the problem. I originally thought it was a redis problem, because of the error message, but it was not. Removing the redis conf settings from settings.conf did not do anything. The only way to solve the problem was to disable the app using occ app:disable previewgenerator

Here's the log:

{
  "reqId": "7mugNd3ki2cFu7W0SOwc",
  "level": 3,
  "time": "2023-09-14T09:27:47+08:00",
  "remoteAddr": "13.40.122.214",
  "user": "--",
  "app": "core",
  "method": "GET",
  "url": "/index.php/apps/files/preview-service-worker.js",
  "message": "NOAUTH Authentication required.",
  "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:109.0) Gecko/20100101 Firefox/117.0",
  "version": "27.0.2.1",
  "exception": {
    "Exception": "RedisException",
    "Message": "NOAUTH Authentication required.",
    "Code": 0,
    "Trace": [
      {
        "file": "/var/www/nextcloud/lib/private/Memcache/Redis.php",
        "line": 72,
        "function": "get",
        "class": "Redis",
        "type": "->"
      },
      {
        "file": "/var/www/nextcloud/lib/private/L10N/Factory.php",
        "line": 346,
        "function": "get",
        "class": "OC\\Memcache\\Redis",
        "type": "->"
      },
      {
        "file": "/var/www/nextcloud/lib/private/L10N/Factory.php",
        "line": 483,
        "function": "findAvailableLanguages",
        "class": "OC\\L10N\\Factory",
        "type": "->"
      },
      {
        "file": "/var/www/nextcloud/lib/private/L10N/Factory.php",
        "line": 219,
        "function": "getLanguageFromRequest",
        "class": "OC\\L10N\\Factory",
        "type": "->"
      },
      {
        "file": "/var/www/nextcloud/lib/public/Util.php",
        "line": 227,
        "function": "findLanguage",
        "class": "OC\\L10N\\Factory",
        "type": "->"
      },
      {
        "file": "/var/www/nextcloud/lib/private/legacy/OC_Template.php",
        "line": 111,
        "function": "addTranslations",
        "class": "OCP\\Util",
        "type": "::"
      },
      {
        "file": "/var/www/nextcloud/lib/private/legacy/OC_Template.php",
        "line": 77,
        "function": "initTemplateEngine",
        "class": "OC_Template",
        "type": "::"
      },
      {
        "file": "/var/www/nextcloud/lib/private/legacy/OC_Template.php",
        "line": 321,
        "function": "__construct",
        "class": "OC_Template",
        "type": "->"
      },
      {
        "file": "/var/www/nextcloud/index.php",
        "line": 74,
        "function": "printExceptionErrorPage",
        "class": "OC_Template",
        "type": "::"
      }
    ],
    "File": "/var/www/nextcloud/lib/private/Memcache/Redis.php",
    "Line": 72,
    "CustomMessage": "--"
  }
}```

Start by tracing how Redis settings from config.php and settings.conf reach the previewgenerator app, comparing that path with the Redis connection shown in lib/private/Memcache/Redis.php. Reproduce the authentication failure and confirm that updated Redis settings are honored without causing 503 responses when previewgenerator is enabled.