Android clients do not display data

Nextcloud version: 20.0.0.9
Operating system and version: Debian 10
Apache or nginx version: nginx/1.18.0
PHP version: 7.4

The issue you are facing:
You won’t see any files on the Nextcloud Android app. It doesn’t matter if you have a new connected account or an already connected account.

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

  1. Log on to the Nextcoud instance.
  2. Open the Nextcloud App

The output of your Nextcloud log in Admin > Logging:

{"reqId":"Lty38F6YaIRKczNpsl82","level":4,"time":"06.10.2020 13:13:19","remoteAddr":"80.187.106.146","user":"1e9fb3c8-a990-1036-844e-33b12343625b","app":"webdav","method":"PROPFIND","url":"/remote.php/webdav/","message":{"Exception":"TypeError","Message":"Argument 1 passed to OC\\Group\\Manager::getUserGroupIds() must implement interface OCP\\IUser, null given, called in /var/www/DOMAIN/lib/private/Share20/DefaultShareProvider.php on line 884","Code":0,"Trace":[{"file":"/var/www/DOMAIN/lib/private/Share20/DefaultShareProvider.php","line":884,"function":"getUserGroupIds","class":"OC\\Group\\Manager","type":"->"},{"file":"/var/www/DOMAIN/lib/private/Share20/Manager.php","line":1333,"function":"getSharedWith","class":"OC\\Share20\\DefaultShareProvider","type":"->"},{"file":"/var/www/DOMAIN/apps/dav/lib/Connector/Sabre/Node.php","line":323,"function":"getSharedWith","class":"OC\\Share20\\Manager","type":"->"},{"file":"/var/www/DOMAIN/apps/dav/lib/Connector/Sabre/FilesPlugin.php","line":372,"function":"getNoteFromShare","class":"OCA\\DAV\\Connector\\Sabre\\Node","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/dav/lib/DAV/PropFind.php","line":96,"function":"OCA\\DAV\\Connector\\Sabre\\{closure}","class":"OCA\\DAV\\Connector\\Sabre\\FilesPlugin","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/var/www/DOMAIN/apps/dav/lib/Connector/Sabre/FilesPlugin.php","line":374,"function":"handle","class":"Sabre\\DAV\\PropFind","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/event/lib/WildcardEmitterTrait.php","line":89,"function":"handleGetProperties","class":"OCA\\DAV\\Connector\\Sabre\\FilesPlugin","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/dav/lib/DAV/Server.php","line":1063,"function":"emit","class":"Sabre\\DAV\\Server","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/dav/lib/DAV/Server.php","line":989,"function":"getPropertiesByNode","class":"Sabre\\DAV\\Server","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/dav/lib/DAV/Server.php","line":1678,"function":"getPropertiesIteratorForPath","class":"Sabre\\DAV\\Server","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/dav/lib/DAV/Server.php","line":1661,"function":"writeMultiStatus","class":"Sabre\\DAV\\Server","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/dav/lib/DAV/CorePlugin.php","line":363,"function":"generateMultiStatus","class":"Sabre\\DAV\\Server","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/event/lib/WildcardEmitterTrait.php","line":89,"function":"httpPropFind","class":"Sabre\\DAV\\CorePlugin","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/dav/lib/DAV/Server.php","line":474,"function":"emit","class":"Sabre\\DAV\\Server","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/dav/lib/DAV/Server.php","line":251,"function":"invokeMethod","class":"Sabre\\DAV\\Server","type":"->"},{"file":"/var/www/DOMAIN/3rdparty/sabre/dav/lib/DAV/Server.php","line":319,"function":"start","class":"Sabre\\DAV\\Server","type":"->"},{"file":"/var/www/DOMAIN/apps/dav/appinfo/v1/webdav.php","line":84,"function":"exec","class":"Sabre\\DAV\\Server","type":"->"},{"file":"/var/www/DOMAIN/remote.php","line":167,"args":["/var/www/DOMAIN/apps/dav/appinfo/v1/webdav.php"],"function":"require_once"}],"File":"/var/www/DOMAIN/lib/private/Group/Manager.php","Line":325,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Android) Nextcloud-android/3.13.1","version":"20.0.0.9"}

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

<?php
$CONFIG = array (
  'passwordsalt' => 'SALT',
  'trusted_domains' => 
  array (
    0 => 'DOMAIN',
  ),
  'datadirectory' => '/home/_data',
  'skeletondirectory' => '/home/_skeleton',
  'tempdirectory' => '/home/box_temp',
  'dbtype' => 'mysql',
  'version' => '20.0.0.9',
  'dbname' => 'db',
  'dbhost' => 'localhost',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'db',
  'dbpassword' => 'PW',
  'mysql.utf8mb4' => true,
  'installed' => true,
  'loglevel' => 3,
  'logdateformat' => 'd.m.Y H:i:s',
  'logtimezone' => 'Europe/Berlin',
  'log_rotate_size' => 10 * 1024 * 1024,
  'maintenance' => false,
  'secret' => 'SECRET',
  'ldapIgnoreNamingRules' => false,
  'forcessl' => true,
  'mail_smtpmode' => 'smtp',
  'mail_from_address' => 'ADRESS',
  'mail_domain' => 'OTHERDOMAIN',
  'mail_smtphost' => 'mail.OTHERDOMAIN',
  'mail_smtpport' => '25',
  'ldapProviderFactory' => '\\OCA\\User_LDAP\\LDAPProviderFactory',
  'ldapUserCleanupInterval' => 51,
  'lost_password_link' => 'https://www.OTHERDOMAIN/de/user/password',
  'instanceid' => 'oc4zv99hzps1',
  'mail_sendmailmode' => 'smtp',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_smtpauth' => 1,
  'mail_smtpname' => '',
  'mail_smtppassword' => '',
  'overwrite.cli.url' => 'https://DOMAIN',
  'memcache.local' => '\\OC\\Memcache\\Redis',
  'memcache.distributed' => '\\OC\\Memcache\\Redis',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'redis' => 
  array (
    'host' => '/var/run/redis/redis-server.sock',
    'port' => 0,
    'timeout' => 0.0,
  ),
  'default_language' => 'de',
  'default_locale' => 'de',
  'trashbin_retention_obligation' => 'auto, 60',
  'versions_retention_obligation' => 'auto, 60',
  'theme' => 'THEME',
  'simpleSignUpLink' => 'false',
  'activity_expire_days' => '90',
);

The output of your Apache/nginx/system log in /var/log/____:

nothing that has to do with this error

Since the app now also crashes with an error message, I reported this directly to github