Error on Nextcloud OpenID Connect login with Authelia

I am currently trying to set up and use Authelia idendity providers with OpenID Connect as a single login provider for several different applications.

The first application I want to add is Nextcloud. To do this, I follow the instructions on the Authelia documentation page: https://www.authelia.com/integration/openid-connect/nextcloud

But the problem I'm facing now is that I always get an error message saying that the redirect URL I specified in the configuration is not valid, even though I used the official URL from the Authelia docs.

Authelia Error:

{
  "error": "invalid_request",
  "error_description": "The request is missing a required parameter, 
      includes an invalid parameter value, includes a parameter more than once, 
      or is otherwise malformed. The 'redirect_uri' parameter does not match 
      any of the OAuth 2.0 Client's pre-registered redirect urls."
}

Authelia Configuration:
(Replaced my server address by: "example.com")

jwt_secret: ac5Tmfbue44TxPTrCpCWNNZKm7AmvBS6
default_redirection_url: https://example.com

server:
  host: 0.0.0.0
  port: 9091

log:
  level: debug

totp:
  issuer: example.com

authentication_backend:
  file:
    path: /config/users_database.yml

access_control:
  default_policy: one_factor
  rules:
    - domain: auth.example.com
      policy: bypass
    - domain:
        - example.com
        - proxy.example.com
        - nextcloud.example.com
      policy: one_factor

identity_providers:
  oidc:
    hmac_secret: GhuVkMctBBFratABE6fMUacCWKhGgNa23SuDuN62Ug6vxQhJJTsLXca3ZdXyuL7n
    issuer_private_key: {KEY}
    access_token_lifespan: 1h
    authorize_code_lifespan: 1m
    id_token_lifespan: 1h
    refresh_token_lifespan: 90m
    enable_client_debug_messages: false
    enforce_pkce: public_clients_only
    cors:
      endpoints:
        - authorization
        - token
        - revocation
        - introspection
      allowed_origins:
        - https://example.com
      allowed_origins_from_client_redirect_uris: false
    clients:
      - id: nextcloud
        secret: nextcloud_client_secret
        public: false
        authorization_policy: one_factor
        scopes:
          - openid
          - profile
          - groups
        redirect_uris:
          - https://nextcloud.example.com/apps/oidc_login/oidc
        userinfo_signing_algorithm: none

session:
  name: authelia_session
  secret: kHT5S9ed8ArygSwyVZWm48Pyjt4qyXg9
  expiration: 3600
  inactivity: 300
  domain: example.com

regulation:
  max_retries: 3
  find_time: 120
  ban_time: 300

storage:
  encryption_key: nqcWgCG22YM6Uttj4GQw5eeNsWKwr4Xm
  local:
    path: /config/db.sqlite3

notifier:
  filesystem:
    filename: /config/notification.txt

Nextcloud Configuration:
(Plugin: OpenID Connect Login)

<?php
$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' => 'owd1s341ok',
  'passwordsalt' => 'Kt7fjLgAGjGMtAdrfdsXR4BEEz5pqzmv',
  'secret' => '64kSvSXtpQ2HUMBEdQTf5NusZ9SnQhBN64kSvSXtpQ2HUe',
  'trusted_domains' => 
  array (
    0 => 'nextcloud.example.com',
  ),
  'datadirectory' => '/var/www/html/data',
  'dbtype' => 'sqlite3',
  'version' => '24.0.1.1',
  'overwrite.cli.url' => 'http://nextcloud.example.com',
  'installed' => true,
  'allow_user_to_change_display_name' => false,
  'lost_password_link' => 'disabled',
  'oidc_login_provider_url' => 'https://auth.example.com',
  'oidc_login_client_id' => 'nextcloud',
  'oidc_login_client_secret' => 'nextcloud_client_secret',
  'oidc_login_auto_redirect' => false,
  'oidc_login_end_session_redirect' => false,
  'oidc_login_button_text' => 'Log in with Authelia',
  'oidc_login_hide_password_form' => false,
  'oidc_login_use_id_token' => true,
  'oidc_login_attributes' => array (
    'id' => 'preferred_username',
    'name' => 'name',
    'mail' => 'email',
    'groups' => 'groups',
  ),
  'oidc_login_default_group' => 'oidc',
  'oidc_login_use_external_storage' => false,
  'oidc_login_scope' => 'openid profile groups',
  'oidc_login_proxy_ldap' => false,
  'oidc_login_disable_registration' => true,
  'oidc_login_redir_fallback' => false,
  'oidc_login_alt_login_page' => 'assets/login.php',
  'oidc_login_tls_verify' => true,
  'oidc_create_groups' => false,
  'oidc_login_webdav_enabled' => false,
  'oidc_login_password_authentication' => false,
  'oidc_login_public_key_caching_time' => 86400,
  'oidc_login_min_time_between_jwks_requests' => 10,
  'oidc_login_well_known_caching_time' => 86400,
  'oidc_login_update_avatar' => false,
);

Does anyone know how I can fix this error or what I need to change to make it work?
(I am using Nginx Proxy Manager as a reverse proxy and Docker to run Nginx Proxy Manager, Authelia and Nextcloud).

Thanks
Ypselon :)

-= Update =-

I noticed that when I try to log in with Authelia, I get the error page with the following URL:

https://auth.example.com/api/oidc/authorization?response_type=code&redirect_uri=http%3A%2F%nextcloud.example.com%2Fapps%2Foidc_login%2Foidc&client_id=nextcloud&nonce=9a2986f054d7044bcb3050ed3c38a1b6&state=9384d2d39924b436f8a0eb6b8bd334ad&scope=openid+profile+groups+openid

I can then simply remove the "redirect_uri" from the query parameters, reload the page and then for some magical reason I am presented with the consent request and after I accept it I get logged in.

Now I'm completely confused, but maybe someone can tell me, based on the behavior, where the problem might be.

Thanks
Ypselon :)