Registration is failing with broken old records in the database

Steps to reproduce

1. Upgrade to NC 20
2. Try to register

Expected behaviour

Registering works fine

Actual behaviour

Fails with 500 error.

Logs:

{"reqId":"7lOAXu5KXnH6rwCveKuM","level":3,"time":"2020-10-16T20:02:48+00:00","remoteAddr":"10.244.33.7","user":"--","app":"index","method":"POST","url":"/apps/registration/","message":{"Exception":"Symfony\\Component\\Routing\\Exception\\InvalidParameterException","Message":"Parameter \"secret\" for route \"registration.register.showUserForm\" must match \"[^/]++\" (\"\" given) to generate a corresponding URL.","Code":0,"Trace":[{"file":"/var/www/html/3rdparty/symfony/routing/Generator/UrlGenerator.php","line":160,"function":"doGenerate","class":"Symfony\\Component\\Routing\\Generator\\UrlGenerator","type":"->","args":[{"secret":0,"token":1},{"caller":["registration","RegisterController","showUserForm"],"action":null},[],[["variable","/","[^/]++","token"],["variable","/","[^/]++","secret"],["text","/apps/registration/register"]],{"secret":null,"token":"qKNPiJzzHt"},"registration.register.showUserForm",1,[],[]]},{"file":"/var/www/html/lib/private/Route/Router.php","line":362,"function":"generate","class":"Symfony\\Component\\Routing\\Generator\\UrlGenerator","type":"->","args":["registration.register.showUserForm",{"secret":null,"token":"qKNPiJzzHt"},1]},{"file":"/var/www/html/lib/private/Route/CachingRouter.php","line":60,"function":"generate","class":"OC\\Route\\Router","type":"->","args":["registration.register.showUserForm",{"secret":null,"token":"qKNPiJzzHt"},false]},{"file":"/var/www/html/lib/private/URLGenerator.php","line":83,"function":"generate","class":"OC\\Route\\CachingRouter","type":"->","args":["registration.register.showUserForm",{"secret":null,"token":"qKNPiJzzHt"}]},{"file":"/var/www/html/lib/private/URLGenerator.php","line":95,"function":"linkToRoute","class":"OC\\URLGenerator","type":"->","args":["registration.register.showUserForm",{"secret":null,"token":"qKNPiJzzHt"}]},{"file":"/var/www/html/custom_apps/registration/lib/Service/MailService.php","line":86,"function":"linkToRouteAbsolute","class":"OC\\URLGenerator","type":"->","args":["registration.register.showUserForm",{"secret":null,"token":"qKNPiJzzHt"}]},{"file":"/var/www/html/custom_apps/registration/lib/Controller/RegisterController.php","line":130,"function":"sendTokenByMail","class":"OCA\\Registration\\Service\\MailService","type":"->","args":[{"id":12,"__class__":"OCA\\Registration\\Db\\Registration"}]},{"file":"/var/www/html/lib/private/AppFramework/Http/Dispatcher.php","line":169,"function":"submitEmailForm","class":"OCA\\Registration\\Controller\\RegisterController","type":"->","args":["dimm0@yandex.ru"]},{"file":"/var/www/html/lib/private/AppFramework/Http/Dispatcher.php","line":100,"function":"executeController","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":[{"__class__":"OCA\\Registration\\Controller\\RegisterController"},"submitEmailForm"]},{"file":"/var/www/html/lib/private/AppFramework/App.php","line":152,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":[{"__class__":"OCA\\Registration\\Controller\\RegisterController"},"submitEmailForm"]},{"file":"/var/www/html/lib/private/Route/Router.php","line":308,"function":"main","class":"OC\\AppFramework\\App","type":"::","args":["OCA\\Registration\\Controller\\RegisterController","submitEmailForm",{"__class__":"OC\\AppFramework\\DependencyInjection\\DIContainer"},{"action":null,"_route":"registration.register.submitEmailForm"}]},{"file":"/var/www/html/lib/base.php","line":1009,"function":"match","class":"OC\\Route\\Router","type":"->","args":["/apps/registration/"]},{"file":"/var/www/html/index.php","line":37,"function":"handleRequest","class":"OC","type":"::","args":[]}],"File":"/var/www/html/3rdparty/symfony/routing/Generator/UrlGenerator.php","Line":193,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.121 Safari/537.36 OPR/71.0.3770.228","version":"20.0.0.9"}

Server configuration

Operating system:
Container

Nextcloud version: (see Nextcloud admin page)
20.0.0

Updated from an older Nextcloud/ownCloud or fresh install:
Updated from 19.0.4

Signing status:

Technical information
=====================
The following list covers which files have failed the integrity check. Please read
the previous linked documentation to learn more about the errors and how to fix
them.

Results
=======
- core
	- EXTRA_FILE
		- aria2c.sess

Raw output
==========
Array
(
    [core] => Array
        (
            [EXTRA_FILE] => Array
                (
                    [aria2c.sess] => Array
                        (
                            [expected] => 
                            [current] => 4605678dd6e205b54d85a43a6c7a2e491f5d9e4271965ef5cba6eac811facdcf1296559baebc5d018c15818b9dd3b36aa2dbcc6cc1a6f171cf546cf9dd483950
                        )

                )

        )

)

List of activated apps:

Enabled:
  - accessibility: 1.6.0
  - activity: 2.13.1
  - apporder: 0.11.0
  - bruteforcesettings: 2.0.1
  - camerarawpreviews: 0.7.8
  - cloud_federation_api: 1.3.0
  - comments: 1.10.0
  - contacts: 3.4.0
  - contactsinteraction: 1.1.0
  - dashboard: 7.0.0
  - dav: 1.16.0
  - dicomviewer: 1.2.2
  - federatedfilesharing: 1.10.1
  - federation: 1.10.1
  - files: 1.15.0
  - files_external: 1.11.1
  - files_markdown: 2.3.1
  - files_pdfviewer: 2.0.1
  - files_photospheres: 1.20.0
  - files_rightclick: 0.17.0
  - files_sharing: 1.12.0
  - files_trashbin: 1.10.1
  - files_videoplayer: 1.9.0
  - firstrunwizard: 2.9.0
  - guests: 1.6.0
  - lookup_server_connector: 1.8.0
  - metadata: 0.12.0
  - news: 15.0.4
  - nextcloud_announcements: 1.9.0
  - notes: 4.0.0
  - notifications: 2.8.0
  - oauth2: 1.8.0
  - ocdownloader: 1.7.8
  - password_policy: 1.10.1
  - photos: 1.2.0
  - privacy: 1.4.0
  - provisioning_api: 1.10.0
  - recommendations: 0.8.0
  - registration: 0.5.1
  - serverinfo: 1.10.0
  - settings: 1.2.0
  - sharebymail: 1.10.0
  - sociallogin: 3.4.1
  - support: 1.3.0
  - systemtags: 1.10.0
  - tasks: 0.13.4
  - text: 3.1.0
  - theming: 1.11.0
  - twofactor_backupcodes: 1.9.0
  - twofactor_totp: 5.0.0
  - twofactor_u2f: 6.0.0
  - user_status: 1.0.0
  - viewer: 1.4.0
  - weather_status: 1.0.0
  - workflowengine: 2.2.0
Disabled:
  - admin_audit
  - analytics
  - announcementcenter
  - calendar
  - cookbook
  - dashboardcharts
  - encryption
  - event_update_notification
  - external
  - extract
  - files_3d
  - files_accesscontrol
  - files_versions
  - flowupload
  - gpgmailer
  - groupfolders
  - logreader
  - maps
  - ocr
  - radio
  - survey_client
  - twofactor_nextcloud_notification
  - unsplash
  - updatenotification
  - user_ldap
  - workflow_pdf_converter

The content of config/config.php:

<?php
$CONFIG = array (
  'htaccess.RewriteBase' => '/',
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'memcache.distributed' => '\\OC\\Memcache\\Redis',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'redis' =>
  array (
    'host' => 'nextclouddb',
    'port' => 6379,
    'timeout' => 3,
  ),
  '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' => '',
  'passwordsalt' => '',
  'secret' => '',
  'trusted_domains' =>
  array (
...
  ),
  'datadirectory' => '/var/www/html/data',
  'overwriteprotocol' => 'https',
  'overwrite.cli.url' => '...',
  'dbtype' => 'mysql',
  'version' => '20.0.0.9',
  'dbname' => 'nextclouddb',
  'dbhost' => 'nextclouddb',
  'dbport' => '',
  'dbtableprefix' => '',
  'mysql.utf8mb4' => true,
  'dbuser' => 'nextcloud',
  'dbpassword' => '...',
  'installed' => true,
mail...
  'maintenance' => false,
  'updater.release.channel' => 'stable',
  'loglevel' => 0,
  'enabledPreviewProviders' =>
  array (
    0 => 'OC\\Preview\\PNG',
    1 => 'OC\\Preview\\JPEG',
    2 => 'OC\\Preview\\GIF',
    3 => 'OC\\Preview\\BMP',
    4 => 'OC\\Preview\\XBitmap',
    5 => 'OC\\Preview\\MP3',
    6 => 'OC\\Preview\\TXT',
    7 => 'OC\\Preview\\MarkDown',
    8 => 'OC\\Preview\\TIFF',
  ),
  'twofactor_enforced' => 'false',
  'twofactor_enforced_groups' =>
  array (
  ),
  'twofactor_enforced_excluded_groups' =>
  array (
  ),
  'app_install_overwrite' =>
  array (
    0 => 'camerarawpreviews',
    1 => 'dicomviewer',
    2 => 'radio',
    3 => 'extract',
    4 => 'unsplash',
    5 => 'files_photospheres',
    6 => 'files_3d',
    7 => 'ocdownloader',
    8 => 'registration',
  ),
  'mail_sendmailmode' => 'smtp',
  'mail_smtpsecure' => 'tls',
  'theme' => '',
);

Are you using external storage, if yes which one: local/smb/sftp/...
cephFS

Are you using encryption: yes/no
no

Are you using an external user-backend, if yes which one: LDAP/ActiveDirectory/Webdav/...
no

Browser log

Internal Server Error
The server was unable to complete your request.

If this happens again, please send the technical details below to the server administrator.

More details can be found in the server log.

Start by reproducing the upgrade from Nextcloud 19.0.4 to 20.0.0 and registration failure, then inspect custom_apps/registration/lib/Service/MailService.php and custom_apps/registration/lib/Controller/RegisterController.php alongside the logged Symfony URL generation trace. Determine how the old database record produces the missing route parameter and verify that registration no longer returns a 500 error.