Locked into redirection loop at login

Nextcloud version (eg, 18.0.2): 19.0.3
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04.1
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.41
PHP version (eg, 7.1): 7.4.3

The issue you are facing:
When trying to login into my nextcloud at nextcloud.mydomain.de (https) I get redirected to the login page and the URL in the address bar changes to https://nextcloud.mydomain.de/index.php/login?redirect_url=/index.php/apps/files/
It happens with both admin and user accounts.

I do not recollect what might be the cause of this. My only guess goes toward a conflict of redirections, maybe between the certbot certificate (signed to mydomain.de and *.mydomain.de) and redirection rules in the various config files in /etc/apache2.

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

Steps to replicate it:

  1. Visit nextcloud.mydomain.de (http or https)
  2. Enter valid admin or user credentials, hit login button
  3. Land on login page again

The output of your Nextcloud log in Admin > Logging:

{"reqId":"*","level":2,"time":"2020-08-17T17:33:35+00:00","remoteAddr":"95.91.226.131","user":"--","app":"no app in context","method":"GET","url":"/","message":"Could not detect any host in http:///data/htaccesstest.txt","userAgent":"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:79.0) Gecko/20100101 Firefox/79.0","version":""}
{"reqId":"*","level":2,"time":"2020-08-17T17:33:35+00:00","remoteAddr":"95.91.226.131","user":"--","app":"no app in context","method":"GET","url":"/","message":"Could not detect any host in https:///data/htaccesstest.txt","userAgent":"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:79.0) Gecko/20100101 Firefox/79.0","version":""}
{"reqId":"*","level":3,"time":"2020-08-17T17:33:35+00:00","remoteAddr":"95.91.226.131","user":"--","app":"jsresourceloader","method":"GET","url":"/","message":"Could not find resource js/setup.js to load","userAgent":"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:79.0) Gecko/20100101 Firefox/79.0","version":""}
{"reqId":"*","level":3,"time":"2020-08-17T17:33:35+00:00","remoteAddr":"95.91.226.131","user":"--","app":"PHP","method":"GET","url":"/","message":"chmod(): No such file or directory at /var/www/nextcloud/lib/private/Log/File.php#86","userAgent":"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:79.0) Gecko/20100101 Firefox/79.0","version":""}
{"reqId":"*","level":3,"time":"2020-08-17T17:35:08+00:00","remoteAddr":"40.117.213.15","user":"--","app":"PHP","method":"GET","url":"/","message":"session_start(): A session had already been started - ignoring at /var/www/nextcloud/lib/private/Session/Internal.php#209","userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36 Edge/16.16299","version":""}
{"reqId":"*","level":2,"time":"2020-08-17T17:35:08+00:00","remoteAddr":"40.117.213.15","user":"--","app":"no app in context","method":"GET","url":"/","message":"Could not detect any host in http:///data/htaccesstest.txt","userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36 Edge/16.16299","version":""}
{"reqId":"*","level":2,"time":"2020-08-17T17:35:08+00:00","remoteAddr":"40.117.213.15","user":"--","app":"no app in context","method":"GET","url":"/","message":"Could not detect any host in https:///data/htaccesstest.txt","userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36 Edge/16.16299","version":""}
{"reqId":"*","level":3,"time":"2020-08-17T17:35:08+00:00","remoteAddr":"40.117.213.15","user":"--","app":"jsresourceloader","method":"GET","url":"/","message":"Could not find resource js/setup.js to load","userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36 Edge/16.16299","version":""}
{"reqId":"*","level":3,"time":"2020-08-17T17:36:59+00:00","remoteAddr":"95.91.226.131","user":"--","app":"PHP","method":"POST","url":"/index.php","message":"session_start(): A session had already been started - ignoring at /var/www/nextcloud/lib/private/Session/Internal.php#209","userAgent":"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:79.0) Gecko/20100101 Firefox/79.0","version":""}
{"reqId":"*","level":2,"time":"2020-08-17T17:36:59+00:00","remoteAddr":"95.91.226.131","user":"--","app":"no app in context","method":"POST","url":"/index.php","message":"Could not detect any host in http:///data/htaccesstest.txt","userAgent":"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:79.0) Gecko/20100101 Firefox/79.0","version":""}
{"reqId":"*","level":2,"time":"2020-08-17T17:36:59+00:00","remoteAddr":"95.91.226.131","user":"--","app":"no app in context","method":"POST","url":"/index.php","message":"Could not detect any host in https:///data/htaccesstest.txt","userAgent":"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:79.0) Gecko/20100101 Firefox/79.0","version":""}

(Why the log dates back to 17th August instead of today I don't know)

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

<?php
$CONFIG = array (
  'instanceid' => '*',
  'passwordsalt' => '*',
  'secret' => '*',
  'trusted_domains' =>
  array (
    0 => 'nextcloud.mydomain.de',
    1 => 'SERVER-IP',
  ),
  'datadirectory' => '/home/data/',
  'dbtype' => 'mysql',
  'version' => '19.0.3.1',
  'overwrite.cli.url' => 'https://nextcloud.mydomain.de',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => '*',
  'dbpassword' => '*',
  'installed' => true,
  'maintenance' => false,
  'theme' => '',
  'loglevel' => 2,
);

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

[06/Oct/2020:16:29:20 +0200] "GET / HTTP/1.1" 302 1555 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:81.0) Gecko/20100101 Firefox/81.0"
[06/Oct/2020:16:29:20 +0200] "GET /index.php/login HTTP/1.1" 200 5994 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:81.0) Gecko/20100101 Firefox/81.0"
[06/Oct/2020:16:29:20 +0200] "GET /cron.php HTTP/1.1" 200 940 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:81.0) Gecko/20100101 Firefox/81.0"
[06/Oct/2020:16:29:26 +0200] "POST /index.php/login HTTP/1.1" 303 1412 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:81.0) Gecko/20100101 Firefox/81.0"
[06/Oct/2020:16:29:26 +0200] "GET /index.php/apps/files/ HTTP/1.1" 303 830 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:81.0) Gecko/20100101 Firefox/81.0"
[06/Oct/2020:16:29:27 +0200] "GET /index.php/login?redirect_url=/index.php/apps/files/ HTTP/1.1" 200 6025 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:81.0) Gecko/20100101 Firefox/81.0"
178.73.215.171 - - [06/Oct/2020:16:29:29 +0200] "GET / HTTP/1.0" 200 11239 "-" "-"
[The above entry seems unrelated to my login request]
[06/Oct/2020:16:29:27 +0200] "GET /cron.php HTTP/1.1" 200 940 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:81.0) Gecko/20100101 Firefox/81.0"

Hello,
have you found a solution?
I have the same error.

Unfortunately not. In the end I changed from apache to nginx and from mysql to mariadb and reinstalled nextcloud. It has been working since then.