Nextcloud version (eg, 18.0.2): 18.0.8
Operating system and version (eg, Ubuntu 20.04): Ubuntu 18.04 LTS
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4
PHP version (eg, 7.1): PHP 7.2.24
The issue you are facing:
Everything was working great. LDAP enabled and the old document_server. We wanted to upgrade to 19 because of the new documentserver with Collabora. So I disabled the old document_server, upgraded PHP to 7.4 and run updater.phar. Everything worked great.
I enabled everything and some tests proved, that everything is working. But then I noticed, that my desktop client does not sync anymore. It stays at the “disc”-icon: 
And then someone called me, that a shared folder does not open anymore: After hitting the “Share”-Button next to the folder-name (in the browser) he got “Fehler beim laden der Datei-Daten” what means “Error loading file-data”. A inspection of that request brings up an error "App not installed: "
That only happened with INTERNAL shared files/folders - Internal means: Share with an NC user.
Clicking on the folder itself brings the infinite loading symbol. The inspection console shows a “404” error with no further error message.
I had to revert to NC 18 as this instance in for productive usage. I will clone the system and test further.
Does anyone has similar experiences? I found Nextcloud 19.0.0 Update screwed my installation but without any solution - but sounds similar.
Is this the first time you’ve seen this error? (Y/N): Y
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => '',
'passwordsalt' => '',
'secret' => '',
'trusted_domains' =>
array (
0 => 'nextcloud.julius-kuehn.de',
),
'datadirectory' => '/nextcloud',
'dbtype' => 'mysql',
'version' => '18.0.7.1',
'overwrite.cli.url' => 'http://nextcloud.host.tld',
'dbname' => 'nextcloud',
'dbhost' => '4.3.2.1',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => 'nextcloud',
'dbpassword' => '',
'installed' => true,
'proxy' => '1.2.3.6:8000',
'proxyexclude' =>
array (
0 => '1.2.3.4',
),
'ldapIgnoreNamingRules' => false,
'ldapProviderFactory' => 'OCA\\User_LDAP\\LDAPProviderFactory',
'mail_smtpmode' => 'sendmail',
'mail_sendmailmode' => 'smtp',
'mail_from_address' => 'cloud',
'mail_domain' => '----',
'skeletondirectory' => '',
'maintenance' => false,
'theme' => '',
'loglevel' => 2,
'memcache.local' => '\\OC\\Memcache\\APCu',
'app_install_overwrite' =>
array (
0 => 'mindmap_app',
),
'filelocking.enabled' => true,
'memcache.locking' => '\\OC\\Memcache\\Redis',
'redis' =>
array (
'host' => '/var/run/redis/redis-server.sock',
'port' => 0,
'timeout' => 0.0,
),
'updater.release.channel' => 'stable',
);