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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): 17.0.1
Operating system and version (eg, Ubuntu 17.04): Arch Linux
Apache or nginx version (eg, Apache 2.4.25): 2.4.41
PHP version (eg, 7.1): 7.2
The issue you are facing:
I upgraded from Nextcloud production 16.0.6 (I think) to Nextcloud stable 17.0.1 as provided by my package manager. I want to get updates together with my system updates and so on.
Everything works fine, but, however, no login password works anymore. I have tried this for several user accounts, some passwords I had even saved in keepass. But all passwords are rejected.
How is that happening? Is there a way to restore the old passwords, as I guess their hashes are still in the database, just invalidated somehow. Is there a way to check that? It would be a pity if I had to assign new passwords to each user.
In the nextcloud log, I get a warning about a bruteforce attempt, even if I cleared the oc_bruteforce_attempt table before.
Any ideas are very appreciated. Thanks in advance!
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
I can’t state replicatability, as I can’t downgrade and then do the upgrade again
output of your Nextcloud log in Admin > Logging:
{"reqId":"M7h3HIG2jC6ueUJUXrhK","level":2,"time":"2019-12-16T21:05:59+00:00","remoteAddr":"217.83.83.199","user":"--","app":"no app in context","method":"POST","url":"\/nextcloud\/index.php\/login","message":"Login failed: ipad (Remote IP: 217.83.83.199)","userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:71.0) Gecko\/20100101 Firefox\/71.0","version":"17.0.1.1"}
{"reqId":"M7h3HIG2jC6ueUJUXrhK","level":1,"time":"2019-12-16T21:05:59+00:00","remoteAddr":"217.83.83.199","user":"--","app":"core","method":"POST","url":"\/nextcloud\/index.php\/login","message":"Bruteforce attempt from \"217.83.83.199\" detected for action \"login\".","userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:71.0) Gecko\/20100101 Firefox\/71.0","version":"17.0.1.1"}
{"reqId":"oa58MLBrujRsxUTvaSlT","level":2,"time":"2019-12-16T21:08:11+00:00","remoteAddr":"134.99.174.1","user":"--","app":"no app in context","method":"POST","url":"\/nextcloud\/index.php\/login","message":"Login failed: ipad (Remote IP: 134.99.174.1)","userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:71.0) Gecko\/20100101 Firefox\/71.0","version":"17.0.1.1"}
{"reqId":"oa58MLBrujRsxUTvaSlT","level":1,"time":"2019-12-16T21:08:11+00:00","remoteAddr":"134.99.174.1","user":"--","app":"core","method":"POST","url":"\/nextcloud\/index.php\/login","message":"Bruteforce attempt from \"134.99.174.1\" detected for action \"login\".","userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:71.0) Gecko\/20100101 Firefox\/71.0","version":"17.0.1.1"}
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => 'xxxxx',
'passwordsalt' => 'xxxxx',
'secret' => 'xxxxx',
'trusted_domains' =>
array (
0 => '192.168.2.101',
1 => 'xxxxx',
),
'datadirectory' => '/usr/share/webapps/nextcloud/data',
'overwrite.cli.url' => 'xxxxx',
'dbtype' => 'mysql',
'version' => '17.0.1.1',
'dbname' => 'nextcloud',
'dbhost' => 'xxxxx',
'dbport' => '',
'dbtableprefix' => 'oc_',
'dbuser' => 'nextcloud',
'dbpassword' => 'xxxxx',
'installed' => true,
'memcache.local' => '\\OC\\Memcache\\APCu',
'mail_smtpmode' => 'smtp',
'mail_smtpauthtype' => 'LOGIN',
'mail_from_address' => 'xxxxx',
'mail_domain' => 'xxxxx',
'mail_smtpsecure' => 'tls',
'mail_smtpauth' => 1,
'mail_smtphost' => 'xxxxx',
'mail_smtpport' => '587',
'mail_smtpname' => 'xxxxx',
'mail_smtppassword' => 'xxxxx',
'apps_paths' =>
array (
0 =>
array (
'path' => '/usr/share/webapps/nextcloud/apps',
'url' => '/apps',
'writable' => false,
),
1 =>
array (
'path' => '/usr/share/webapps/nextcloud/apps2',
'url' => '/apps2',
'writable' => true,
),
),
'loglevel' => 0,
'maintenance' => false,
'updater.release.channel' => 'stable',
'theme' => '',
);
The output of your Apache/nginx/system log in /var/log/____:
134.99.174.1 - - [16/Dec/2019:22:17:43 +0100] "POST /nextcloud/index.php/login HTTP/1.1" 303 -
134.99.174.1 - - [16/Dec/2019:22:17:43 +0100] "GET /nextcloud/index.php/login?user=ipad HTTP/1.1" 200 12541
134.99.174.1 - - [16/Dec/2019:22:17:44 +0100] "GET /nextcloud/apps/files_pdfviewer/css/style.css?v=ca9f0d77-0 HTTP/1.1" 200 346
134.99.174.1 - - [16/Dec/2019:22:17:44 +0100] "GET /nextcloud/core/css/guest.css?v=9b96d7f1-0 HTTP/1.1" 200 19902
134.99.174.1 - - [16/Dec/2019:22:17:44 +0100] "GET /nextcloud/index.php/apps/theming/styles?v=0 HTTP/1.1" 200 987
134.99.174.1 - - [16/Dec/2019:22:17:44 +0100] "GET /nextcloud/core/img/favicon.ico HTTP/1.1" 200 3262
134.99.174.1 - - [16/Dec/2019:22:17:44 +0100] "GET /nextcloud/core/img/favicon-touch.png HTTP/1.1" 200 2918
134.99.174.1 - - [16/Dec/2019:22:17:44 +0100] "GET /nextcloud/index.php/svg/core/logo/logo?color=fff&v=1 HTTP/1.1" 200 321