I just ran docker-compose pull && docker-compose up -d, which updated my Nextcloud instance from 19.0.3 to 20.x. This however left me with an "Internal Server Error", that I cannot resolve.

I am using the configuration found at https://github.com/nextcloud/docker/tree/master/.examples/docker-compose/with-nginx-proxy-self-signed-ssl/mariadb/fpm.

I have also tried to use the very basic Apache installation, which leaves me with the same issue.

// Edit:
The logs for app simply shows "GET /index.php" 500, whereas when using the basic Apache version:

{
    "reqId": "zvJbAmLWqPJleaRZBapb",
    "level": 3,
    "time": "2020-10-17T11:46:17+00:00",
    "remoteAddr": "192.168.0.104",
    "user": "--",
    "app": "PHP",
    "method": "GET",
    "url": "/",
    "message": {
        "Exception": "Error",
        "Message": "touch(): Utime failed: Permission denied at /var/www/html/lib/private/Config.php#242",
        "Code": 0,
        "Trace": [{
            "function": "onError",
            "class": "OC\\\\Log\\\\ErrorHandler",
            "type": "::",
            "args": [2, "touch(): Utime failed: Permission denied", "/var/www/html/lib/private/Config.php", 242, {
                "content": "<?php\\n$CONFIG = array (\\n  'memcache.local' => '\\\\\\\\OC\\\\\\\\Memcache\\\\\\\\APCu',\\n  'apps_paths' => \\n  array (\\n    0 => \\n    array (\\n      'path' => '/var/www/html/apps',\\n      'url' => '/apps',\\n      'writable' => false,\\n    ),\\n    1 => \\n    array (\\n      'path' => '/var/www/html/custom_apps',\\n      'url' => '/custom_apps',\\n      'writable' => true,\\n    ),\\n  ),\\n  'memcache.distributed' => '\\\\\\\\OC\\\\\\\\Memcache\\\\\\\\Redis',\\n  'memcache.locking' => '\\\\\\\\OC\\\\\\\\Memcache\\\\\\\\Redis',\\n  'redis' => \\n  array (\\n    'host' => 'redis',\\n    'password' => false,\\n    'port' => 6379,\\n  ),\\n  'instanceid' => 'ocd8sshgm2jk',\\n);\\n"
            }]
        }, {
            "file": "/var/www/html/lib/private/Config.php",
            "line": 242,
            "function": "touch",
            "args": ["/var/www/html/config/config.php"]
        }, {
            "file": "/var/www/html/lib/private/Config.php",
            "line": 134,
            "function": "writeData",
            "class": "OC\\\\Config",
            "type": "->",
            "args": []
        }, {
            "file": "/var/www/html/lib/private/SystemConfig.php",
            "line": 111,
            "function": "setValue",
            "class": "OC\\\\Config",
            "type": "->",
            "args": ["instanceid", "ocd8sshgm2jk"]
        }, {
            "file": "/var/www/html/lib/private/legacy/OC_Util.php",
            "line": 1167,
            "function": "setValue",
            "class": "OC\\\\SystemConfig",
            "type": "->",
            "args": ["instanceid", "ocd8sshgm2jk"]
        }, {
            "file": "/var/www/html/lib/base.php",
            "line": 421,
            "function": "getInstanceId",
            "class": "OC_Util",
            "type": "::",
            "args": []
        }, {
            "file": "/var/www/html/lib/base.php",
            "line": 650,
            "function": "initSession",
            "class": "OC",
            "type": "::",
            "args": []
        }, {
            "file": "/var/www/html/lib/base.php",
            "line": 1092,
            "function": "init",
            "class": "OC",
            "type": "::",
            "args": []
        }, {
            "file": "/var/www/html/index.php",
            "line": 35,
            "args": ["/var/www/html/lib/base.php"],
            "function": "require_once"
        }],
        "File": "/var/www/html/lib/private/Log/ErrorHandler.php",
        "Line": 91,
        "CustomMessage": "--"
    },
    "userAgent": "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:81.0) Gecko/20100101 Firefox/81.0",
    "version": ""
}

However, docker-compose exec app ls -al /var/www/html shows:

drwxrwxrwx   14 www-data root          4096 Oct 17 10:28 .
drwxrwxr-x    1 www-data root          4096 Oct  8 19:48 ..
-rw-r--r--    1 www-data root          3032 Oct 17 10:28 .htaccess
-rw-r--r--    1 www-data root           101 Oct 17 10:28 .user.ini
drwxr-xr-x   41 www-data root          4096 Oct 17 10:28 3rdparty
-rw-r--r--    1 www-data root         17234 Oct 17 10:28 AUTHORS
-rw-r--r--    1 www-data root         34520 Oct 17 10:28 COPYING
drwxr-xr-x   47 www-data root          4096 Oct 17 10:28 apps
drwxr-xr-x    2 www-data root          4096 May  1 18:17 config
-rw-r--r--    1 www-data root          3967 Oct 17 10:28 console.php
drwxr-xr-x   22 www-data root          4096 Oct 17 10:28 core
-rw-r--r--    1 www-data root          5140 Oct 17 10:28 cron.php
drwxr-xr-x   13 www-data root          4096 Oct 17 08:58 custom_apps
drwxrwx---    7 www-data root          4096 Oct 16 17:45 data
-rw-r--r--    1 www-data root           156 Oct 17 10:28 index.html
-rw-r--r--    1 www-data root          2960 Oct 17 10:28 index.php
drwxr-xr-x    6 www-data root          4096 Oct 17 10:28 lib
-rwxr-xr-x    1 www-data root           283 Oct 17 10:28 occ
drwxr-xr-x    2 www-data root          4096 Oct 17 10:28 ocm-provider
drwxr-xr-x    2 www-data root          4096 Oct 17 10:28 ocs
drwxr-xr-x    2 www-data root          4096 Oct 17 10:28 ocs-provider
-rw-r--r--    1 www-data root          3102 Oct 17 10:28 public.php
-rw-r--r--    1 www-data root          5332 Oct 17 10:28 remote.php
drwxr-xr-x    4 www-data root          4096 Oct 17 10:28 resources
-rw-r--r--    1 www-data root            26 Oct 17 10:28 robots.txt
-rw-r--r--    1 www-data root          2379 Oct 17 10:28 status.php
drwxr-xr-x    3 www-data root          4096 May  1 18:14 themes
-rw-r--r--    1 www-data root           362 Oct 17 10:28 version.php

Which looks correct?

Any help would be very much appreciated.