Errors since upgrading underlying OS to Ubuntu 21.10

Nextcloud version (eg, 20.0.5): 22.1.1
Operating system and version (eg, Ubuntu 20.04): Ubuntu 21.10
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.48
PHP version (eg, 7.4): 8.0

The issue you are facing:
Since upgrading my OS to Ubuntu 21.10 with PHP 8.0 I am seeing the following error message when I view the Security and Setup Warnings in Settings;

Last background job execution ran yesterday. Something seems wrong.

My background job is set to use system Cron to run cron.php every 5 minutes. If I open a terminal and run the command;

sudo -u www-data php /var/www/nextcloud/cron.php

I receive this error;

PHP Warning: PHP Startup: Unable to load dynamic library ‘apc.so’ (tried: /usr/lib/php/20200930/apc.so (/usr/lib/php/20200930/apc.so: cannot open shared object file: No such file or directory), /usr/lib/php/20200930/apc.so.so (/usr/lib/php/20200930/apc.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
OC\HintException: [0]: Memcache \OC\Memcache\APCu not available for local cache (Is the matching PHP module installed and enabled?)

Is this the first time you’ve seen this error? (Y/N): Y it worked fine before the upgrade

Steps to replicate it:

  1. Upgrade in place Ubuntu from 21.04 to 21.10 (This upgrades PHP to 8.0
  2. Clean up any residual php 7.4 modules
  3. Install any missing php 8.0 modules

The output of your Nextcloud log in Admin > Logging:

The latest entry is 3 weeks ago, long before I made these changes

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

<?php
$CONFIG = array (
  'instanceid' => 'xxxxxxxxx',
  'passwordsalt' => 'xxxxxxxxxxxxxxxxxxxx',
  'secret' => 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx',
  'trusted_domains' => 
  array (
    0 => 'localhost',
    1 => '127.0.0.1',
    2 => '172.xx.xx.xx',
    3 => 'xxxxx.local',
    4 => 'xxxxx.xxx.xx.xx',
  ),
  'datadirectory' => '/raid5/nextcloud/data',
  'dbtype' => 'mysql',
  'version' => '22.1.1.2',
  'overwrite.cli.url' => 'https://xxxx.xxx.xx.xx/nextcloud',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost:3306',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'nc_dbuser',
  'dbpassword' => 'xxxxxxxxxxxx',
  'installed' => true,
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'htaccess.RewriteBase' => '/nextcloud',
  'trashbin_retention_obligation' => 'auto, 120',
  'versions_retention_obligation' => 'auto, 120',
  'mail_smtpmode' => 'smtp',
  'mail_smtpsecure' => 'tls',
  'mail_sendmailmode' => 'smtp',
  'mail_from_address' => 'xxxxxx',
  'mail_domain' => 'xxx.xx.xx',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_smtpauth' => 1,
  'mail_smtphost' => 'smtp.gmail.com',
  'mail_smtpport' => '587',
  'mail_smtpname' => 'xxxxxxx@xxx.xx.xx',
  'mail_smtppassword' => 'xxxxxxxxxxxxxxxx',
  'maintenance' => false,
  'theme' => '',
  'loglevel' => 2,
);

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

PHP Warning:  PHP Startup: Unable to load dynamic library 'apc.so' (tried: /usr/lib/php/20200930/apc.so (/usr/lib/php/20200930/apc.so: cannot open shared object file: No such file or directory), /usr/lib/php/20200930/apc.so.so (/usr/lib/php/20200930/apc.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
[Sun Oct 17 21:47:48.562145 2021] [mpm_prefork:notice] [pid 876] AH00163: Apache/2.4.48 (Ubuntu) OpenSSL/1.1.1l configured -- resuming normal operations
[Sun Oct 17 21:47:48.562273 2021] [core:notice] [pid 876] AH00094: Command line: '/usr/sbin/apache2'
[Sun Oct 17 22:57:26.637683 2021] [mpm_prefork:notice] [pid 876] AH00170: caught SIGWINCH, shutting down gracefully
PHP Warning:  PHP Startup: Unable to load dynamic library 'apc.so' (tried: /usr/lib/php/20200930/apc.so (/usr/lib/php/20200930/apc.so: cannot open shared object file: No such file or directory), /usr/lib/php/20200930/apc.so.so (/usr/lib/php/20200930/apc.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
[Sun Oct 17 22:57:42.664518 2021] [mpm_prefork:notice] [pid 877] AH00163: Apache/2.4.48 (Ubuntu) OpenSSL/1.1.1l configured -- resuming normal operations
[Sun Oct 17 22:57:42.665012 2021] [core:notice] [pid 877] AH00094: Command line: '/usr/sbin/apache2'