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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, 18.0.2): 18.0.4
Operating system and version (eg, Ubuntu 20.04): Debian 10
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.38 (Debian)
PHP version (eg, 7.1): PHP 7.3.14-1~deb10u1
The issue you are facing:
My Nextcloud error log contains repeated entries like the one shown below. They show that a lock file used by Joplin on my phone is not accepted for upload. Yet upload is repeatedly attempted.
I have a somewhat similar problem with a file called ClatterAsCisternFills.aac even though .aac is a standard extension for sound recordings. This is compounded by the fact that I tried to delete the file locally (it no longer matters) but now Nextcloud is logging repeated failures for the deletion.
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
- Install Joplin on phone or create file with .aac extension
- Link to Nextcloud
- Watch Nextcloud log
The output of your Nextcloud log in Admin > Logging:
{"reqId":"L54oljcKz6na8rOxCfZT","level":2,"time":"2020-06-01T16:48:48+00:00","remoteAddr":"79.79.41.34","user":"xxxxx","app":"ransomware_protection","method":"PROPFIND","url":"/remote.php/webdav/Joplin/.lock/","message":"Prevented upload of Joplin/.lock because it matches extension pattern \".lock\"","userAgent":"Mozilla/5.0 (Android) Nextcloud-android/3.11.1","version":"18.0.4.2"}
{"reqId":"DJYzMdEITTMQzwqx0R9p","level":4,"time":"2020-06-01T18:19:16+00:00","remoteAddr":"79.79.41.34","user":"xxxxxx","app":"webdav","method":"DELETE","url":"/remote.php/dav/files/xxxx/yyyyyyy/ClatterAsCisternFills.aac","message":{"Exception":"OCA\\DAV\\Connector\\Sabre\\Exception\\Forbidden","Message":"Ransomware file detected","Code":0,"Trace":[{"file":"/var/www/nextcloud/public_html/3rdparty/sabre/dav/lib/DAV/Tree.php","line":179,"function":"delete","class":"OCA\\DAV\\Connector\\Sabre\\File","type":"->","args":[]},{"file":"/var/www/nextcloud/public_html/3rdparty/sabre/dav/lib/DAV/CorePlugin.php","line":287,"function":"delete","class":"Sabre\\DAV\\Tree","type":"->","args":["files/xxxxx/yyyyyyy/ClatterAsCisternFills.aac"]},{"function":"httpDelete","class":"Sabre\\DAV\\CorePlugin","type":"->","args":[{"absoluteUrl":"https://example.com/remote.php/dav/files/xxxxx/yyyyyyyy/ClatterAsCisternFills.aac","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"/var/www/nextcloud/public_html/3rdparty/sabre/event/lib/EventEmitterTrait.php","line":105,"function":"call_user_func_array","args":[[{"__class__":"Sabre\\DAV\\CorePlugin"},"httpDelete"],[{"absoluteUrl":"https://example.com/remote.php/dav/files/xxxx/yyyyyyy/ClatterAsCisternFills.aac","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]]},{"file":"/var/www/nextcloud/public_html/3rdparty/sabre/dav/lib/DAV/Server.php","line":479,"function":"emit","class":"Sabre\\Event\\EventEmitter","type":"->","args":["method:DELETE",[{"absoluteUrl":"https://example.com/remote.php/dav/files/xxxxx/yyyyyyy/ClatterAsCisternFills.aac","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]]},{"file":"/var/www/nextcloud/public_html/3rdparty/sabre/dav/lib/DAV/Server.php","line":254,"function":"invokeMethod","class":"Sabre\\DAV\\Server","type":"->","args":[{"absoluteUrl":"https://example.com/remote.php/dav/files/xxxxx/yyyyyyy/ClatterAsCisternFills.aac","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"/var/www/nextcloud/public_html/apps/dav/lib/Server.php","line":319,"function":"exec","class":"Sabre\\DAV\\Server","type":"->","args":[]},{"file":"/var/www/nextcloud/public_html/apps/dav/appinfo/v2/remote.php","line":35,"function":"exec","class":"OCA\\DAV\\Server","type":"->","args":[]},{"file":"/var/www/nextcloud/public_html/remote.php","line":165,"args":["/var/www/nextcloud/public_html/apps/dav/appinfo/v2/remote.php"],"function":"require_once"}],"File":"/var/www/nextcloud/public_html/apps/dav/lib/Connector/Sabre/File.php","Line":455,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Linux) mirall/2.6.4git (Nextcloud)","version":"18.0.4.2"}
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => 'ocdx0h2o6yoh',
'passwordsalt' => 'M+gfICgKTJoXVAIF66Sxz1oFlbSRbI',
'secret' => '*******',
'trusted_domains' =>
array (
0 => 'cloud.example.com',
1 => 'example.com',
),
'datadirectory' => '/var/lib/nextcloud',
'overwrite.cli.url' => 'https://cloud.example.com',
'dbtype' => 'mysql',
'version' => '18.0.4.2',
'dbname' => 'nextcloud',
'dbhost' => 'localhost',
'dbport' => '',
'dbtableprefix' => 'oc_',
'dbuser' => 'nextcloud',
'dbpassword' => '*******',
'installed' => true,
'memcache.local' => '\\OC\\Memcache\\APCu',
// 'memcache.local' => '\\OC\\Memcache\\Redis',
'maintenance' => false,
'filelocking.enabled' => true,
'memcache.locking' => '\\OC\\Memcache\\Redis',
'redis' =>
array (
'host' => '/var/run/redis/redis.sock',
'port' => 0,
'timeout' => 0.0,
),
'updater.secret' => '*******',
'theme' => '',
'loglevel' => 2,
'mysql.utf8mb4' => true,
'updater.release.channel' => 'stable',
'auth.bruteforce.protection.enabled' => false,
);
The output of your Apache/nginx/system log in /var/log/____:
Nothing relevant