Database connection error after Ubuntu release upgrade

Nextcloud version (eg, 19.0.4.2:
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04
Apache or nginx version (eg, Apache 2.4.25): 2.4.41
PHP version (eg, 7.1): 7.4

The issue you are facing:

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

The output of your Nextcloud log in Admin > Logging:

{"reqId":"E9jWUQ6wDNEzlSpLII0o","level":3,"time":"November 03, 2020 {"reqId":"7iZyF7So8oG4bJ2v11k2","level":3,"time":"November 03, 2020 10:10:12","remoteAddr":"192.168.170.188","user":"--","app":"index","method":"POST","url":"/login/v2/poll","message":{"Exception":"Doctrine\\DBAL\\DBALException","Message":"Failed to connect to the database: An exception occurred in driver: could not find driver","Code":0,"Trace":[{"file":"/var/www/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php","line":889,"function":"connect","class":"OC\\DB\\Connection","type":"->"},{"file":"/var/www/nextcloud/lib/private/DB/Connection.php","line":194,"function":"executeQuery","class":"Doctrine\\DBAL\\Connection","type":"->"},{"file":"/var/www/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Query/QueryBuilder.php","line":200,"function":"executeQuery","class":"OC\\DB\\Connection","type":"->"},{"file":"/var/www/nextcloud/lib/private/DB/QueryBuilder/QueryBuilder.php","line":216,"function":"execute","class":"Doctrine\\DBAL\\Query\\QueryBuilder","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppConfig.php","line":331,"function":"execute","class":"OC\\DB\\QueryBuilder\\QueryBuilder","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppConfig.php","line":109,"function":"loadConfigValues","class":"OC\\AppConfig","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppConfig.php","line":287,"function":"getApps","class":"OC\\AppConfig","type":"->"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_App.php","line":883,"function":"getValues","class":"OC\\AppConfig","type":"->"},{"file":"/var/www/nextcloud/lib/private/Server.php","line":665,"function":"getAppVersions","class":"OC_App","type":"::"},{"file":"/var/www/nextcloud/3rdparty/pimple/pimple/src/Pimple/Container.php","line":118,"function":"OC\\{closure}","class":"OC\\Server","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/var/www/nextcloud/lib/private/ServerContainer.php","line":124,"function":"offsetGet","class":"Pimple\\Container","type":"->"},{"file":"/var/www/nextcloud/lib/private/Server.php","line":1704,"function":"query","class":"OC\\ServerContainer","type":"->"},{"file":"/var/www/nextcloud/lib/private/Server.php","line":856,"function":"getMemCacheFactory","class":"OC\\Server","type":"->"},{"file":"/var/www/nextcloud/3rdparty/pimple/pimple/src/Pimple/Container.php","line":118,"function":"OC\\{closure}","class":"OC\\Server","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/var/www/nextcloud/lib/private/ServerContainer.php","line":124,"function":"offsetGet","class":"Pimple\\Container","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppFramework/Utility/SimpleContainer.php","line":163,"function":"query","class":"OC\\ServerContainer","type":"->"},{"file":"/var/www/nextcloud/3rdparty/pimple/pimple/src/Pimple/Container.php","line":114,"function":"OC\\AppFramework\\Utility\\{closure}","class":"OC\\AppFramework\\Utility\\SimpleContainer","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/var/www/nextcloud/lib/private/ServerContainer.php","line":124,"function":"offsetGet","class":"Pimple\\Container","type":"->"},{"file":"/var/www/nextcloud/lib/private/Server.php","line":1894,"function":"query","class":"OC\\ServerContainer","type":"->"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_App.php","line":347,"function":"getAppManager","class":"OC\\Server","type":"->"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_Util.php","line":1438,"function":"getEnabledApps","class":"OC_App","type":"::"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_Util.php","line":716,"function":"needUpgrade","class":"OC_Util","type":"::"},{"file":"/var/www/nextcloud/lib/base.php","line":661,"function":"checkServer","class":"OC_Util","type":"::"},{"file":"/var/www/nextcloud/lib/base.php","line":1094,"function":"init","class":"OC","type":"::"},{"file":"/var/www/nextcloud/index.php","line":35,"args":["/var/www/nextcloud/lib/base.php"],"function":"require_once"}],"File":"/var/www/nextcloud/lib/private/DB/Connection.php","Line":67,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Linux) mirall/2.6.4git (Nextcloud)","version":"19.0.4.2"}
{"reqId":"7iZyF7So8oG4bJ2v11k2","level":3,"time":"November 03, 2020 10:10:12","remoteAddr":"192.168.170.188","user":"--","app":"core","method":"POST","url":"/login/v2/poll","message":{"Exception":"Doctrine\\DBAL\\DBALException","Message":"Failed to connect to the database: An exception occurred in driver: could not find driver","Code":0,"Trace":[{"file":"/var/www/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php","line":889,"function":"connect","class":"OC\\DB\\Connection","type":"->"},{"file":"/var/www/nextcloud/lib/private/DB/Connection.php","line":194,"function":"executeQuery","class":"Doctrine\\DBAL\\Connection","type":"->"},{"file":"/var/www/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Query/QueryBuilder.php","line":200,"function":"executeQuery","class":"OC\\DB\\Connection","type":"->"},{"file":"/var/www/nextcloud/lib/private/DB/QueryBuilder/QueryBuilder.php","line":216,"function":"execute","class":"Doctrine\\DBAL\\Query\\QueryBuilder","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppConfig.php","line":331,"function":"execute","class":"OC\\DB\\QueryBuilder\\QueryBuilder","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppConfig.php","line":109,"function":"loadConfigValues","class":"OC\\AppConfig","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppConfig.php","line":287,"function":"getApps","class":"OC\\AppConfig","type":"->"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_App.php","line":883,"function":"getValues","class":"OC\\AppConfig","type":"->"},{"file":"/var/www/nextcloud/lib/private/Server.php","line":665,"function":"getAppVersions","class":"OC_App","type":"::"},{"file":"/var/www/nextcloud/3rdparty/pimple/pimple/src/Pimple/Container.php","line":118,"function":"OC\\{closure}","class":"OC\\Server","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/var/www/nextcloud/lib/private/ServerContainer.php","line":124,"function":"offsetGet","class":"Pimple\\Container","type":"->"},{"file":"/var/www/nextcloud/lib/private/Server.php","line":1704,"function":"query","class":"OC\\ServerContainer","type":"->"},{"file":"/var/www/nextcloud/lib/private/Server.php","line":856,"function":"getMemCacheFactory","class":"OC\\Server","type":"->"},{"file":"/var/www/nextcloud/3rdparty/pimple/pimple/src/Pimple/Container.php","line":118,"function":"OC\\{closure}","class":"OC\\Server","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/var/www/nextcloud/lib/private/ServerContainer.php","line":124,"function":"offsetGet","class":"Pimple\\Container","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppFramework/Utility/SimpleContainer.php","line":163,"function":"query","class":"OC\\ServerContainer","type":"->"},{"file":"/var/www/nextcloud/3rdparty/pimple/pimple/src/Pimple/Container.php","line":114,"function":"OC\\AppFramework\\Utility\\{closure}","class":"OC\\AppFramework\\Utility\\SimpleContainer","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/var/www/nextcloud/lib/private/ServerContainer.php","line":124,"function":"offsetGet","class":"Pimple\\Container","type":"->"},{"file":"/var/www/nextcloud/lib/private/Server.php","line":1894,"function":"query","class":"OC\\ServerContainer","type":"->"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_App.php","line":347,"function":"getAppManager","class":"OC\\Server","type":"->"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_Util.php","line":1438,"function":"getEnabledApps","class":"OC_App","type":"::"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_Util.php","line":716,"function":"needUpgrade","class":"OC_Util","type":"::"},{"file":"/var/www/nextcloud/lib/base.php","line":661,"function":"checkServer","class":"OC_Util","type":"::"},{"file":"/var/www/nextcloud/lib/base.php","line":1094,"function":"init","class":"OC","type":"::"},{"file":"/var/www/nextcloud/index.php","line":35,"args":["/var/www/nextcloud/lib/base.php"],"function":"require_once"}],"File":"/var/www/nextcloud/lib/private/DB/Connection.php","Line":67,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Linux) mirall/2.6.4git (Nextcloud)","version":"19.0.4.2"}
{"reqId":"7iZyF7So8oG4bJ2v11k2","level":3,"time":"November 03, 2020 10:10:12","remoteAddr":"192.168.170.188","user":"--","app":"core","method":"POST","url":"/login/v2/poll","message":{"Exception":"Doctrine\\DBAL\\DBALException","Message":"Failed to connect to the database: An exception occurred in driver: could not find driver","Code":0,"Trace":[{"file":"/var/www/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php","line":889,"function":"connect","class":"OC\\DB\\Connection","type":"->"},{"file":"/var/www/nextcloud/lib/private/DB/Connection.php","line":194,"function":"executeQuery","class":"Doctrine\\DBAL\\Connection","type":"->"},{"file":"/var/www/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Query/QueryBuilder.php","line":200,"function":"executeQuery","class":"OC\\DB\\Connection","type":"->"},{"file":"/var/www/nextcloud/lib/private/DB/QueryBuilder/QueryBuilder.php","line":216,"function":"execute","class":"Doctrine\\DBAL\\Query\\QueryBuilder","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppConfig.php","line":331,"function":"execute","class":"OC\\DB\\QueryBuilder\\QueryBuilder","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppConfig.php","line":109,"function":"loadConfigValues","class":"OC\\AppConfig","type":"->"},{"file":"/var/www/nextcloud/lib/private/AppConfig.php","line":287,"function":"getApps","class":"OC\\AppConfig","type":"->"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_App.php","line":883,"function":"getValues","class":"OC\\AppConfig","type":"->"},{"file":"/var/www/nextcloud/lib/private/TemplateLayout.php","line":164,"function":"getAppVersions","class":"OC_App","type":"::"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_Template.php","line":185,"function":"__construct","class":"OC\\TemplateLayout","type":"->"},{"file":"/var/www/nextcloud/lib/private/Template/Base.php","line":132,"function":"fetchPage","class":"OC_Template","type":"->"},{"file":"/var/www/nextcloud/lib/private/legacy/OC_Template.php","line":334,"function":"printPage","class":"OC\\Template\\Base","type":"->"},{"file":"/var/www/nextcloud/index.php","line":63,"function":"printExceptionErrorPage","class":"OC_Template","type":"::"}],"File":"/var/www/nextcloud/lib/private/DB/Connection.php","Line":67,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Linux) mirall/2.6.4git (Nextcloud)","version":"19.0.4.2"}

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

<?php
$CONFIG = array (
  'instanceid' => 'ocakkxvjpmct',
  'passwordsalt' => 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
  'secret' => 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
  'trusted_domains' =>
  array (
    0 => '192.168.170.28',
    1 => 'nextcloud.example.loc',
    2 => 'nextcloud.example.com',
  ),
  'trusted_proxies' =>
  array (
    0 => '192.168.170.26',
  ),
  'datadirectory' => '/mnt/data/nextcloud',
  'dbtype' => 'mysql',
  'version' => '19.0.4.2',
  'overwrite.cli.url' => 'http://nextcloud.example.com',
  'overwritehost' => 'nextcloud.example.com',
  'overwriteprotocol' => 'https',
  'htaccess.RewriteBase' => '/',
  'dbname' => 'nextcloud',
  'dbhost' => '192.168.170.21:3306',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'nextcloud',
  'dbpassword' => 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
  'installed' => true,
  'theme' => '',
  'mail_smtpmode' => 'smtp',
  'mail_sendmailmode' => 'smtp',
  'mail_smtpauth' => 1,
  'mail_smtpname' => 'admin@example.com',
  'mail_smtppassword' => 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
  'mail_smtphost' => 'smtp.ionos.de',
  'mail_domain' => 'example.com',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_smtpsecure' => 'tls',
  'mail_from_address' => 'admin',
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'log_type' => 'file',
  'logfile' => '/var/log/nextcloud.log',
  'logfilemode' => 416,
  'loglevel' => '2',
  'logdateformat' => 'F d, Y H:i:s',
  'mail_smtpport' => '587',
  'updater.secret' => 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
  'updater.release.channel' => 'stable',
  'app_install_overwrite' =>
  array (
    0 => 'spreed',
    1 => 'onlyoffice',
    2 => 'files_fulltextsearch_tesseract',
  ),
  'maintenance' => true,
);

I have made an update from Ubuntu 19.10 to 20.04
PHP7.3 was uninstalled.
After I changed everything to PHP7.4 I still get the following error.
Extract from the Nextcloud log.

"Message":"Failed to connect to the database: An exception occurred in driver: could not find driver"

Your db type is mysql.
So please install php-mysql module.
Use the web to find help. That is an release common topic for ubuntu+php-mysql.

Hey,
the module php-mysql is installed

I removed it with apt-get purge php-mysql and installed it again.
Now it works.

Thanks a lot.