Update available, but updater has all checks 'passed' before the update is started

Nextcloud version (eg, 12.0.2): 14.0.10
Operating system and version (eg, Ubuntu 17.04): Debian GNU/Linux 9.9 (stretch)
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.25
PHP version (eg, 7.1): PHP-FPM 7.3 (but I had the same symptoms with v. 7.0)

The issue you are facing:
I have successfully updated the instance from v13 to v14.0.10, but now I want to upgrade to v15.0.7 and the update just doesn’t start. The Admin settings pane tells me there’s an update available for me, in the Stable channel. Opening the updater, shows the regular updater index-page, but immediately all checks are shown green. I can click the button to continue with the update, but that just enables the button further down the page and does nothing else.

I have also tried using the updater.phar, but that shows roughly the same sequence: all ‘steps that will be executed’ are highlighted in green before I even confirmed the (y/N) question to continue…

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

Steps to replicate it:

  1. Try to update using the web updater or using the command line.

The output of your Nextcloud log in Admin > Logging:

(empty, even with 'debug level' selected)

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

<?php
$CONFIG = array (
  'passwordsalt' => <mine>,
  'secret' => <mine>,
  'trusted_domains' => 
  array (
    0 => '<domain>',
    1 => 'cloud.<domain>',
    2 => 'backup.<domain>',
  ),
  'datadirectory' => '/var/www/nextcloud/data',
  'overwrite.cli.url' => 'https://cloud.<domain>',
  'dbtype' => 'pgsql',
  'version' => '14.0.10.0',
  'dbname' => 'nextcloud',
  'dbhost' => '/var/run/postgresql',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'nextcloud',
  'installed' => true,
  'instanceid' => '<mine>',
  'enable_previews' => true,
  'enabledPreviewProviders' => 
  array (
    0 => 'OC\\Preview\\PNG',
    1 => 'OC\\Preview\\JPEG',
    2 => 'OC\\Preview\\GIF',
    3 => 'OC\\Preview\\BMP',
    4 => 'OC\\Preview\\XBitmap',
    5 => 'OC\\Preview\\MarkDown',
    6 => 'OC\\Preview\\MP3',
    7 => 'OC\\Preview\\TXT',
    8 => 'OC\\Preview\\Illustrator',
    9 => 'OC\\Preview\\Movie',
    10 => 'OC\\Preview\\MSOffice2003',
    11 => 'OC\\Preview\\MSOffice2007',
    12 => 'OC\\Preview\\MSOfficeDoc',
    13 => 'OC\\Preview\\OpenDocument',
    14 => 'OC\\Preview\\PDF',
    15 => 'OC\\Preview\\Photoshop',
    16 => 'OC\\Preview\\Postscript',
    17 => 'OC\\Preview\\StarOffice',
    18 => 'OC\\Preview\\SVG',
    19 => 'OC\\Preview\\TIFF',
    20 => 'OC\\Preview\\Font',
  ),
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'filelocking.enabled' => 'true',
  'redis' => 
  array (
    'host' => '/var/run/redis/redis.sock',
    'port' => 0,
    'timeout' => 0,
    'password' => '<mine>',
  ),
  'ldapIgnoreNamingRules' => false,
  'ldapProviderFactory' => '\\OCA\\User_LDAP\\LDAPProviderFactory',
  'maintenance' => false,
  'loglevel' => 2,
  'updater.release.channel' => 'stable',
  'updater.secret' => '<mine>',
);

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

[proxy_fcgi:error] [pid 26211] [client <mine>:52816] AH01071: Got error 'PHP message: PHP Warning:  "continue" targeting switch is equivalent to "break". Did you mean to use "continue 2"? in /var/www/nextcloud/3rdparty/sabre/vobject/lib/Component/VCard.php on line 509'
[authz_core:error] [pid 26199] [client <mine>:47404] AH01630: client denied by server configuration: /var/www/nextcloud/data/.ocdata
[authz_core:error] [pid 27593] [client <mine>:59600] AH01630: client denied by server configuration: /var/www/nextcloud/data/.ocdata