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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, 20.0.5): 19.0.12 - 19.0.12.0
Operating system and version (eg, Ubuntu 20.04): Linux 4.9.159-vs2.3.9.8-vs+ #4 SMP Wed Aug 28 15:05:40 PDT 2019 x86_64
Apache or nginx version (eg, Apache 2.4.25): Apache (cgi-fcgi) (This is what the Nextcloud system report gives me.)
PHP version (eg, 7.4): 7.4.12
The issue you are facing:
For weeks, Nextcloud is telling me it can’t synch a file. The file does not exist. The error keeps popping up. See screenshot.
https://babasprojects.com/nextcloud/index.php/s/eKFnsCjPQgf9NLD
When pressing “Show details”, nothing happens. When I check the log, there is nothing about this issue.
Is this the first time you’ve seen this error? (Y/N): N
Steps to replicate it:
I can not replicate it. It’s just this one file about which Nextcloud complains. I removed the file, yet the complaints keep coming.
The output of your Nextcloud log in Admin > Logging:
I do not understand where I am to look for this. I do not see an ‘Admin’ menu option anywhere, nor do I see an ‘Admin’ folder on the server.
I cleaned the file /nextcloud/data/nextcloud.log at the start of the day and, after getting the notification I mention a number of times again, this log is literally completely empty.
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
I do not have a config.php file in this location. I do have it in /nextcloud/config/config.php.
What is considered not identifiable, in this file? I’ve removed most values.
<?php
$CONFIG = array (
'instanceid' => '',
'passwordsalt' => '',
'secret' => '',
'trusted_domains' =>
array (
0 => '',
),
'datadirectory' => '',
'overwrite.cli.url' => '',
'dbtype' => 'mysql',
'version' => '19.0.12.0',
'dbname' => '',
'dbhost' => '',
'dbport' => '',
'dbtableprefix' => 'oc_',
'dbuser' => '',
'dbpassword' => '',
'installed' => true,
'maintenance' => false,
'theme' => '',
'loglevel' => 2,
'mail_smtpmode' => 'sendmail',
'mail_from_address' => '',
'mail_domain' => '',
'mail_smtpauthtype' => 'PLAIN',
'mail_smtphost' => '',
'mail_smtpport' => '465',
'mail_smtpsecure' => 'ssl',
);
The output of your Apache/nginx/system log in /var/log/____:
I don’t appear to have permission to download this file.