Files are (suddenly) missing. How do I track down their deletion?

Nextcloud version (eg, 12.0.2): 11.0.6
Operating system and version (eg, Ubuntu 17.04): Ubuntu 16.04
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.18
PHP version (eg, 7.1): Php 7.0.22

The issue you are facing:

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

Steps to replicate it:

  1. Upgrade nextcloud from owncloud
  2. wait

The output of your Nextcloud log in Admin > Logging:

|Warning|no app in context|Missing expected parameters in change user hook|2018-03-09T10:27:36+0100|
|---|---|---|---|
|Warning|no app in context|Missing expected parameters in change user hook|2018-03-09T10:27:36+0100|
|Warning|no app in context|Missing expected parameters in change user hook|2018-03-09T10:27:36+0100|
|Warning|no app in context|Missing expected parameters in change user hook|2018-03-09T10:27:36+0100|
|Warning|no app in context|Missing expected parameters in change user hook|2018-03-09T10:27:36+0100|
|Warning|no app in context|Missing expected parameters in change user hook|2018-03-09T10:27:36+0100|
|Error|PHP|fclose() expects parameter 1 to be resource, boolean given at /var/www/nextcloud/lib/private/Files/Storage/Local.php#218|2018-03-09T10:27:36+0100|
|Error|PHP|fread() expects parameter 1 to be resource, boolean given at /var/www/nextcloud/lib/private/Files/Storage/Local.php#217|2018-03-09T10:27:36+0100|
|Error|PHP|fopen(/mnt/infocloud/appdata_oc3xzc0toftd/avatar/slevtchenko/avatar.jpg): failed to open stream: No such file or directory at /var/www/nextcloud/lib/private/Files/Storage/Local.php#216|2018-03-09T10:27:36+0100|
|Error|PHP|filesize(): stat failed for /mnt/infocloud/appdata_oc3xzc0toftd/avatar/slevtchenko/avatar.jpg at /var/www/nextcloud/lib/private/Files/Storage/Local.php#211|2018-03-09T10:27:36+0100|
|Warning|no app in context|Missing expected parameters in change user hook|2018-03-09T10:27:36+0100|
|Warning|no app in context|Missing expected parameters in change user hook|2018-03-09T10:27:36+0100|
|Warning|no app in context|Missing expected parameters in change user hook|2018-03-09T10:27:35+0100|

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

{
    "system": {
        "updatechecker": false,
        "instanceid": "oc3xzc0toftd",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "***server ip***",
            "infocloud.domain.de"
        ],
        "datadirectory": "\/mnt\/infocloud\/",
        "overwrite.cli.url": "https:\/\/***server ip***\/",
        "dbtype": "mysql",
        "version": "11.0.6.1",
        "logtimezone": "Europe\/Berlin",
        "installed": true,
        "maintenance": false,
        "loglevel": 0,
        "theme": "owncloud-theme-domain",
        "mail_from_address": "infocloud",
        "mail_smtpmode": "smtp",
        "mail_domain": "domain.de",
        "dbname": "infocloud_new",
        "dbhost": "***database host ip***",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "ldapIgnoreNamingRules": false,
        "singleuser": false,
        "mail_smtphost": "***smtp host ip***",
        "mail_smtpport": "25",
        "ldapProviderFactory": "\\OCA\\User_LDAP\\LDAPProviderFactory",
        "auth.bruteforce.protection.enabled": false
    }
}

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

nothing interesting...

A user confronted me that he lost some files on his client. We confirmed they are also not present on disk layer on the server.

I see the files being create in table oc_activity but no deletion.
The sync logfiles on his client dont show any activity on these files so they could be removed long time ago.
How do I track down where the files are gone?