No folders neither files visible

Nextcloud version (eg, 20.0.5): 21.0.2
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04.2 LTS
Apache or nginx version (eg, Apache 2.4.25): 2.4.41
PHP version (eg, 7.4): 7.4.3

The issue you are facing:
EmptyNextcloud
The menus are there but the folder view is empty

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

Steps to replicate it:

  1. I was not able to update this nextcloud from 20 to 21. There was written, the instance is on the newest version.
  2. I have updated the instance manually according to Upgrade manually — Nextcloud latest Administration Manual latest documentation I did not stop the webserver service because there are to many pages on my server. Instead I have renamed the folder name.
  3. I have copied the default config.php instead of my. Then I have copied my after regonizing that it is not working.
  4. I have recognized that there did was woring something with the permissions of my data folder. So I have corrected them according to the manual.
  5. My plugin group folder was deleted by the upgrade process. I have copied it later manually inside of the app-folder because it did not appear in the appstore. After copying it, I was able to update it and I did so.
  6. Now I can see in the admin settings that the group folder settings are correctly set like in the past and they should be all there.
  7. After opening the folder area like in the screenshot I can not see any files or folders neither from the group folder neither from me.
  8. The situation is the same with Chrome, Firefox and Edge
    9 The situation is the same with a test user which I have just created

The output of your Nextcloud log in Admin > Logging:

Info	no app in context	Deprecated event type for OCA\Files::loadAdditionalScripts: Symfony\Component\EventDispatcher\GenericEvent is used		2021-06-29T23:10:19+0200
Info	no app in context	Deprecated event type for OCA\Files::loadAdditionalScripts: Symfony\Component\EventDispatcher\GenericEvent is used		2021-06-29T23:10:19+0200
Info	no app in context	Deprecated event type for OCA\Files::loadAdditionalScripts: Symfony\Component\EventDispatcher\GenericEvent is used		2021-06-29T23:10:19+0200
Info	no app in context	Deprecated event type for OCA\Files::loadAdditionalScripts: Symfony\Component\EventDispatcher\GenericEvent is used		2021-06-29T23:10:19+0200
Info	no app in context	Deprecated event type for OCA\Files::loadAdditionalScripts: Symfony\Component\EventDispatcher\GenericEvent is used		2021-06-29T23:10:19+0200
Info	no app in context	Deprecated event type for OCA\Files::loadAdditionalScripts: Symfony\Component\EventDispatcher\GenericEvent is used		2021-06-29T23:10:19+0200
Info	no app in context	Deprecated event type for OCA\Files::loadAdditionalScripts: Symfony\Component\EventDispatcher\GenericEvent is used		2021-06-29T23:10:19+0200

The full log file is attached under https://we.tl/t-DbijQNJnFd

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

<?php
$CONFIG = array (
  'instanceid' => 'abc',
  'passwordsalt' => 'abcd',
  'secret' => 'abcde',
  'trusted_domains' => 
  array (
    0 => 'cloud.medien-bildung.info',
  ),
  'datadirectory' => '/var/kunden/webs/rainer/cloud.medien-bildung.info/data',
  'overwrite.cli.url' => 'http://cloud.medien-bildung.info',
  'dbtype' => 'mysql',
  'version' => '21.0.2.1',
  'dbname' => 'sql123',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'sql123',
  'dbpassword' => 'Test123',
  'logtimezone' => 'UTC',
  'installed' => true,
  'mail_smtpmode' => 'smtp',
  'mail_from_address' => 'cloud',
  'mail_domain' => 'medien-bildung.info',
  'mail_smtphost' => 'localhost',
  'theme' => '',
  'loglevel' => 0,
  'maintenance' => false,
  'mail_smtpauth' => 1,
  'mail_smtpname' => 'cloud@medien-bildung.info',
  'mail_smtppassword' => 'Test123',
  'encryption.legacy_format_support' => true,
  'encryption.key_storage_migrated' => false,
);

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

[Tue Jun 29 18:37:55.953961 2021] [access_compat:error] [pid 29955] [client 80.155.47.132:62168] AH01797: client denied by server configuration: /var/kunden/webs/rainer/cloud.mzlw.de/data/.ocdata
[Tue Jun 29 19:47:32.868062 2021] [authz_core:error] [pid 35282] [client 109.75.177.194:44822] AH01630: client denied by server configuration: /var/kunden/webs/rainer/rainerwiederstein.de/.user.ini, referer: https://rainerwiederstein.de/.user.ini
[Tue Jun 29 21:48:22.218090 2021] [access_compat:error] [pid 45422] [client 80.155.47.132:59739] AH01797: client denied by server configuration: /var/kunden/webs/rainer/cloud.medien-bildung.info/data/.ocdata
[Tue Jun 29 21:48:41.804836 2021] [access_compat:error] [pid 147069] [client 80.155.47.132:49391] AH01797: client denied by server configuration: /var/kunden/webs/rainer/cloud.medien-bildung.info/data/.ocdata
[Tue Jun 29 21:49:39.829587 2021] [access_compat:error] [pid 147069] [client 80.155.47.132:54297] AH01797: client denied by server configuration: /var/kunden/webs/rainer/cloud.medien-bildung.info/data/.ocdata
[Tue Jun 29 21:50:10.410424 2021] [access_compat:error] [pid 147112] [client 80.155.47.132:55861] AH01797: client denied by server configuration: /var/kunden/webs/rainer/cloud.medien-bildung.info/data/.ocdata
[Tue Jun 29 21:50:39.704915 2021] [fcgid:warn] [pid 147361] [client 80.155.47.132:58876] mod_fcgid: stderr: PHP Warning:  require(/var/kunden/webs/rainer/cloud.medien-bildung.info/3rdparty/composer/../symfony/polyfill-mbstring/bootstrap.php): failed to open stream: No such file or directory in /var/kunden/webs/rainer/cloud.medien-bildung.info/3rdparty/composer/autoload_real.php on line 66
[Tue Jun 29 21:50:39.705204 2021] [fcgid:warn] [pid 147361] [client 80.155.47.132:58876] mod_fcgid: stderr: PHP Fatal error:  require(): Failed opening required '/var/kunden/webs/rainer/cloud.medien-bildung.info/3rdparty/composer/../symfony/polyfill-mbstring/bootstrap.php' (include_path='/var/kunden/webs/rainer/cloud.medien-bildung.info/3rdparty/pear/archive_tar:/var/kunden/webs/rainer/cloud.medien-bildung.info/3rdparty/pear/console_getopt:/var/kunden/webs/rainer/cloud.medien-bildung.info/3rdparty/pear/pear-core-minimal/src:/var/kunden/webs/rainer/cloud.medien-bildung.info/3rdparty/pear/pear_exception:/var/kunden/webs/rainer/cloud.medien-bildung.info/apps') in /var/kunden/webs/rainer/cloud.medien-bildung.info/3rdparty/composer/autoload_real.php on line 66
[Tue Jun 29 21:52:33.961104 2021] [access_compat:error] [pid 147361] [client 80.155.47.132:49685] AH01797: client denied by server configuration: /var/kunden/webs/rainer/cloud.medien-bildung.info/data/.ocdata
[Tue Jun 29 21:54:52.173159 2021] [access_compat:error] [pid 147112] [client 80.155.47.132:49814] AH01797: client denied by server configuration: /var/kunden/webs/rainer/cloud.medien-bildung.info/data/.ocdata **This appears in the other nextcloud, too. But the other is working**

There did just come out a new version of the nextcloud.
With this version everything is solved.

Had exactly the same issue after upgrading from 21.0.2 to 21.0.3. Had to revert back to 21.0.2 as I was not able to find a solution and had to get a working situation again.