Upgrade from package fails

Nextcloud version (eg, 12.0.2): 15
Operating system and version (eg, Ubuntu 17.04): Debian Stretch
Apache or nginx version (eg, Apache 2.4.25):
PHP version (eg, 7.1):

The issue you are facing:
Installed from package, when updating it reports:

Setting up nextcloud-server (15.0.11-1~stretch) ...
Conf nextcloud already enabled
Need upgrade ? true
Nextcloud or one of the apps require upgrade - only a limited number of commands are available
You may use your browser or the occ upgrade command to do the upgrade
An unhandled exception has been thrown:
ParseError: syntax error, unexpected '?' in /var/www/nextcloud/apps/files_external_gdrive/vendor/symfony/console/Helper/ProgressBar.php:99

Stack trace:
#0 /var/www/nextcloud/lib/composer/composer/ClassLoader.php(322): Composer\Autoload\includeFile('/var/www/nextcl...')
#1 [internal function]: Composer\Autoload\ClassLoader->loadClass('Symfony\\Compone...')
#2 /var/www/nextcloud/core/Command/Upgrade.php(106): spl_autoload_call('Symfony\\Compone...')
#3 /var/www/nextcloud/3rdparty/symfony/console/Command/Command.php(255): OC\Core\Command\Upgrade->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#4 /var/www/nextcloud/3rdparty/symfony/console/Application.php(946): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#5 /var/www/nextcloud/3rdparty/symfony/console/Application.php(248): Symfony\Component\Console\Application->doRunCommand(Object(OC\Core\Command\Upgrade), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#6 /var/www/nextcloud/3rdparty/symfony/console/Application.php(148): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#7 /var/www/nextcloud/lib/private/Console/Application.php(213): Symfony\Component\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#8 /var/www/nextcloud/console.php(96): OC\Console\Application->run()
#9 /var/www/nextcloud/occ(11): require_once('/var/www/nextcl...')
#10 {main} **dpkg:** error processing package nextcloud-server (--configure):

 subprocess installed post-installation script returned error exit status 1

Errors were encountered while processing:

 nextcloud-server

E: Sub-process /usr/bin/dpkg returned an error code (1)

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

Steps to replicate it:

  1. apt upgrade

The output of your Nextcloud log in Admin > Logging:

You are using a fallback implementation of the intl extension. Installing the native one is highly recommended instead. at /var/www/nextcloud/3rdparty/patchwork/utf8/src/Patchwork/Utf8/Bootup/intl.php#18

I don’t see an “intl extension” anywhere in my lists of install extensions. Nothing comes up when I search either.

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

<?php
$CONFIG = array (
  'logfile' => '/var/log/nextcloud/nextcloud.log',
  'upgrade.disable-web' => 'true',
  'instanceid' => 'xxx',
  'passwordsalt' => 'xxx',
  'secret' => 'xxx',
  'trusted_domains' => 
  array (
    0 => 'mydomain.com',
  ),
  'datadirectory' => '/var/www/nextcloud/data',
  'dbtype' => 'mysql',
  'version' => '15.0.5.3',
  'overwrite.cli.url' => 'http://mydomain.com/nextcloud',
  'dbname' => 'mydbname',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'mydbuser',
  'dbpassword' => 'mydbpass',
  'installed' => true,
);

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

I'll paste this if it's relevant, but I'm not sure it is.