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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 12.0.2): 15.0.5.3
Operating system and version (eg, Ubuntu 17.04): Ubuntu 16.04.5
Apache or nginx version (eg, Apache 2.4.25): nginx 1.10.3
PHP version (eg, 7.1): 7.0
The issue you are facing: In log I get ton’s of entries “No cache entry found for /appdata_…” and very slow login
Is this the first time you’ve seen this error? (Y/N): started after upgrade from NC13 > NC14 > NC15. Before the upgrade I had trouble with files left locked, due to that error I switched caching from APCu to Redis. However, after that change I did not have these “no cache entry found” errors.
The output of your Nextcloud log in Admin > Logging:
{"reqId":"ICUDDoQz5WZEyOE1Q0he","level":0,"time":"2019-03-11T09:48:16+01:00","remoteAddr":"89.247.160.25","user":"admin","app":"no app in context","method":"GET","url":"\/nextcloud\/apps\/files\/","message":"No cache entry found for \/appdata_ocfkw8d9hskg\/preview\/197521 (storage: local::\/var\/clouddata\/, internalPath: appdata_ocfkw8d9hskg\/preview\/197521)","userAgent":"Mozilla\/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit\/605.1.15 (KHTML, like Gecko) Version\/12.0.1 Safari\/605.1.15","version":"15.0.5.3"}
{"reqId":"ICUDDoQz5WZEyOE1Q0he","level":0,"time":"2019-03-11T09:48:16+01:00","remoteAddr":"89.247.160.25","user":"admin","app":"no app in context","method":"GET","url":"\/nextcloud\/apps\/files\/","message":"No cache entry found for \/appdata_ocfkw8d9hskg\/css\/core\/8aae-9eac-jquery-ui-fixes.css.deps (storage: local::\/var\/clouddata\/, internalPath: appdata_ocfkw8d9hskg\/css\/core\/8aae-9eac-jquery-ui-fixes.css.deps)","userAgent":"Mozilla\/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit\/605.1.15 (KHTML, like Gecko) Version\/12.0.1 Safari\/605.1.15","version":"15.0.5.3"}
{"reqId":"ICUDDoQz5WZEyOE1Q0he","level":0,"time":"2019-03-11T09:48:17+01:00","remoteAddr":"89.247.160.25","user":"admin","app":"no app in context","method":"GET","url":"\/nextcloud\/apps\/files\/","message":"No cache entry found for \/appdata_ocfkw8d9hskg\/preview\/197522 (storage: local::\/var\/clouddata\/, internalPath: appdata_ocfkw8d9hskg\/preview\/197522)","userAgent":"Mozilla\/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit\/605.1.15 (KHTML, like Gecko) Version\/12.0.1 Safari\/605.1.15","version":"15.0.5.3"}
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => 'removed_due_to_privacy',
'passwordsalt' => 'removed_due_to_privacy/l',
'secret' => 'removed_due_to_privacy',
'trusted_domains' =>
array (
0 => 'removed_due_to_privacy',
1 => 'removed_due_to_privacy',
),
'datadirectory' => '/var/clouddata',
'overwrite.cli.url' => 'https://removed_due_to_privacy.com/nextcloud',
'overwriteprotocol' => 'https',
'forcessl' => true,
'dbtype' => 'mysql',
'version' => '15.0.5.3',
'dbname' => 'nextcloud_db',
'dbhost' => 'localhost',
'dbport' => '',
'dbtableprefix' => 'oc_',
'dbuser' => 'nextcloud_db_user',
'dbpassword' => 'removed_due_to_privacy',
'logtimezone' => 'Europe/Berlin',
'installed' => true,
'memcache.local' => '\\OC\\Memcache\\APCu',
'maintenance' => false,
'mail_smtpmode' => 'smtp',
'mail_smtpsecure' => 'ssl',
'mail_from_address' => 'noreply',
'mail_domain' => 'removed_due_to_privacy.com',
'mail_smtpauth' => 1,
'mail_smtpauthtype' => 'LOGIN',
'mail_smtpname' => 'noreply@removed_due_to_privacy.com',
'mail_smtppassword' => 'removed_due_to_privacy',
'mail_smtphost' => 'smtp.removed_due_to_privacy.com',
'mail_smtpport' => '465',
'loglevel' => 0,
'updater.release.channel' => 'production',
'filelocking.enabled' => 'true',
'memcache.locking' => '\\OC\\Memcache\\Redis',
'redis' =>
array (
'host' => '/var/run/redis/redis.sock',
'port' => 0,
'dbindex' => 0,
'timeout' => 1.5,
),
);