0 Class 'JString' not found

[StiffeL]

After upgrading to Joomla 4 me too got an error message in the control panel.

Any advice would be appreciated!

Here is the result from the PFA:

Joomla! Instance :: Joomla! 4.0.2-Stable (Furaha) 24-August-2021
Joomla! Configured :: Yes | Writable (600) |
Configuration Options :: Offline: false | SEF: false | SEF Suffix: false | SEF ReWrite: true | .htaccess/web.config: Yes | GZip: true | Cache: false | CacheTime: 1440 | CacheHandler: redis | CachePlatformPrefix: false | FTP Layer: false | Proxy: false | LiveSite: | Session lifetime: 120 | Session handler: database | Shared sessions: false | SSL: 2 | Error Reporting: developer | Site Debug: true | Language Debug: false | Default Access: 1 | Unicode Slugs: false | dbConnection Type: mysqli | PHP Supports J! 4.0.2: Yes | Database Supports J! 4.0.2: Yes | Database Credentials Present: Yes |

Host Configuration :: OS: Linux | OS Version: 4.18.0-305.lve.el8.x86_64 | Technology: x86_64 | Web Server: LiteSpeed | Encoding: gzip, deflate, br | System TMP Writable: Yes | Free Disk Space : 7035.46 GiB |

PHP Configuration :: Version: 7.4.22 | PHP API: litespeed | Session Path Writable: No | Display Errors: | Error Reporting: 32767 | Log Errors To: | Last Known Error: | Register Globals: | Magic Quotes: | Safe Mode: | Allow url fopen: 1 | Open Base: /home/sldev/:/tmp:/var/tmp:/opt/alt/php73/usr/share/pear/:/dev/urandom:/usr/local/lib/php/:/usr/local/php73/lib/php/ | Uploads: 1 | Max. Upload Size: 16M | Max. POST Size: 16M | Max. Input Time: -1 | Max. Execution Time: 60 | Memory Limit: 128M

Database Configuration :: Version: 10.4.20-MariaDB (Client:10.4.20) | Database Size: 9.81 MiB | #of Tables with config prefix:  114 | #of other Tables:  0 | User Privileges : GRANT ALL

PHP Extensions :: Core (7.4.22) | date (7.4.22) | libxml (7.4.22) | openssl (7.4.22) | pcre (7.4.22) | sqlite3 (7.4.22) | zlib (7.4.22) | bz2 (7.4.22) | calendar (7.4.22) | ctype (7.4.22) | curl (7.4.22) | hash (7.4.22) | filter (7.4.22) | ftp (7.4.22) | gettext (7.4.22) | gmp (7.4.22) | SPL (7.4.22) | iconv (7.4.22) | pcntl (7.4.22) | readline (7.4.22) | Reflection (7.4.22) | session (7.4.22) | standard (7.4.22) | shmop (7.4.22) | SimpleXML (7.4.22) | mbstring (7.4.22) | tokenizer (7.4.22) | xml (7.4.22) | litespeed () | i360 (1.0) | bcmath (7.4.22) | dom (20031129) | fileinfo (7.4.22) | gd (7.4.22) | igbinary (3.2.1) | intl (7.4.22) | json (7.4.22) | exif (7.4.22) | memcache (4.0.5.2) | memcached (3.1.5) | mysqli (7.4.22) | mysqlnd (mysqlnd 7.4.22) | PDO (7.4.22) | pdo_mysql (7.4.22) | pdo_sqlite (7.4.22) | Phar (7.4.22) | posix (7.4.22) | redis (5.3.2) | soap (7.4.22) | sockets (7.4.22) | sodium (7.4.22) | xmlreader (7.4.22) | xmlwriter (7.4.22) | xsl (7.4.22) | zip (1.15.6) | Zend OPcache (7.4.22) | Zend Engine (3.4.0) |
Potential Missing Extensions ::
Disabled Functions :: exec | system | passthru | shell_exec | proc_close | proc_open | dl | popen | show_source | posix_kill | posix_mkfifo | posix_getpwuid | posix_setpgid | posix_setsid | posix_setuid | posix_setgid | posix_seteuid | posix_setegid | posix_uname |

Switch User Environment :: PHP CGI: No | Server SU: No | PHP SU: No | Potential Ownership Issues: No

Core Folders :: images/ (755) | components/ (755) | modules/ (755) | plugins/ (755) | language/ (755) | templates/ (755) | cache/ (755) | logs/ (755) | tmp/ (755) | administrator/components/ (755) | administrator/modules/ (755) | administrator/language/ (755) | administrator/templates/ (755) | administrator/logs/ (755) |

Elevated Permissions (First 10) ::

Database statistics :: Uptime: 1801896 | Threads: 287 | Questions: 5448842307 | Slow queries: 241165 | Opens: 13876166 | Flush tables: 1 | Open tables: 64000 | Queries per second avg: 3023.949 |

Components :: Site ::
Core ::
3rd Party::

Components :: Admin ::
Core :: com_mails (4.0.0) 1 | com_tags (4.0.0) 1 | com_finder (4.0.0) 1 | com_joomlaupdate (4.0.2) 1 | com_ajax (4.0.0) 1 | com_plugins (4.0.0) 1 | com_actionlogs (3.9.0) 1 | com_users (4.0.0) 1 | com_wrapper (4.0.0) 1 | com_fields (4.0.0) 1 | com_modules (4.0.0) 1 | com_redirect (4.0.0) 1 | com_categories (4.0.0) 1 | com_admin (4.0.0) 1 | com_messages (4.0.0) 1 | com_workflow (4.0.0) 1 | com_newsfeeds (4.0.0) 1 | com_cache (4.0.0) 1 | com_templates (4.0.0) 1 | com_menus (4.0.0) 1 | com_checkin (4.0.0) 1 | com_contenthistory (4.0.0) 1 | com_media (3.0.0) 1 | com_postinstall (4.0.0) 1 | com_cpanel (4.0.0) 1 | com_content (4.0.0) 1 | com_associations (4.0.0) 1 | com_config (4.0.0) 1 | com_banners (4.0.0) 1 | com_languages (4.0.0) 1 | com_privacy (3.9.0) 1 | com_login (4.0.0) 1 | com_installer (4.0.0) 1 |
3rd Party::

Modules :: Site ::
Core :: mod_related_items (3.0.0) 1 | mod_footer (3.0.0) 1 | mod_tags_popular (3.1.0) 1 | mod_articles_latest (3.0.0) 1 | mod_users_latest (3.0.0) 1 | mod_wrapper (3.0.0) 1 | mod_whosonline (3.0.0) 1 | mod_syndicate (3.0.0) 1 | mod_stats (3.0.0) 1 | mod_login (3.0.0) 1 | mod_finder (3.0.0) 1 | mod_breadcrumbs (3.0.0) 1 | mod_articles_popular (3.0.0) 1 | mod_menu (3.0.0) 1 | mod_banners (3.0.0) 1 | mod_articles_news (3.0.0) 1 | mod_articles_archive (3.0.0) 1 | mod_custom (3.0.0) 1 | mod_articles_category (3.0.0) 1 | mod_random_image (3.0.0) 1 | mod_languages (3.5.0) 1 | mod_articles_categories (3.0.0) 1 | mod_tags_similar (3.1.0) 1 | mod_feed (3.0.0) 1 |
3rd Party:: Builder (2.6.0-beta.2) 1 |

Modules :: Admin ::
Core :: mod_privacy_dashboard (3.9.0) 1 | mod_frontend (4.0.0) 1 | mod_title (3.0.0) 1 | mod_messages (4.0.0) 1 | mod_submenu (3.0.0) 1 | mod_multilangstatus (3.0.0) 1 | mod_login (3.0.0) 1 | mod_logged (3.0.0) 1 | mod_menu (3.0.0) 1 | mod_toolbar (3.0.0) 1 | mod_user (4.0.0) 1 | mod_latestactions (3.9.0) 1 | mod_latest (3.0.0) 1 | mod_sampledata (3.8.0) ? | mod_privacy_status (4.0.0) 1 | mod_custom (3.0.0) 1 | mod_popular (3.0.0) 1 | mod_version (3.0.0) 1 | mod_loginsupport (4.0.0) 1 | mod_post_installation_messages (4.0.0) 1 | mod_stats_admin (3.0.0) 1 | mod_feed (3.0.0) 1 | mod_quickicon (3.0.0) 1 |
3rd Party::

Libraries ::
Core ::
3rd Party::

Plugins ::
Core :: plg_installer_folderinstaller (3.6.0) 1 | plg_installer_packageinstaller (3.6.0) 1 | plg_installer_urlinstaller (3.6.0) 1 | plg_installer_webinstaller (4.0.0) 1 | plg_installer_override (4.0.0) 1 | plg_webservices_installer (4.0.0) 1 | plg_webservices_users (4.0.0) 1 | plg_webservices_banners (4.0.0) 1 | plg_webservices_newsfeeds (4.0.0) 1 | plg_webservices_tags (4.0.0) 1 | plg_webservices_privacy (4.0.0) 1 | plg_webservices_redirect (4.0.0) 1 | plg_webservices_messages (4.0.0) 1 | plg_webservices_languages (4.0.0) 1 | plg_webservices_modules (4.0.0) 1 | plg_webservices_plugins (4.0.0) 1 | plg_webservices_config (4.0.0) 1 | plg_webservices_content (4.0.0) 1 | plg_webservices_menus (4.0.0) 1 | plg_webservices_templates (4.0.0) 1 | plg_extension_joomla (3.0.0) 1 | plg_extension_finder (4.0.0) 1 | plg_extension_namespacemap (4.0.0) 1 | plg_api-authentication_token (4.0.0) 1 | plg_api-authentication_basic (4.0.0) 1 | plg_twofactorauth_totp (3.2.0) 0 | plg_twofactorauth_yubikey (3.2.0) 0 | plg_workflow_publishing (4.0.0) 1 | plg_workflow_featuring (4.0.0) 1 | plg_workflow_notification (4.0.0) 1 | plg_finder_newsfeeds (3.0.0) 1 | plg_finder_tags (3.0.0) 1 | plg_finder_categories (3.0.0) 1 | plg_finder_contacts (3.0.0) 1 | plg_finder_content (3.0.0) 1 | plg_privacy_message (3.9.0) 1 | plg_privacy_user (3.9.0) 1 | plg_privacy_actionlogs (3.9.0) 1 | plg_privacy_content (3.9.0) 1 | plg_privacy_consents (3.9.0) 1 | plg_authentication_joomla (3.0.0) 1 | plg_authentication_cookie (3.0.0) 1 | plg_authentication_ldap (3.0.0) 0 | plg_user_joomla (3.0.0) 1 | plg_user_contactcreator (3.0.0) 0 | plg_user_profile (3.0.0) 0 | plg_user_token (3.9.0) 1 | plg_user_terms (3.9.0) 0 | plg_system_debug (3.0.0) 1 | plg_system_highlight (3.0.0) 1 | plg_system_privacyconsent (3.9.0) 0 | plg_system_log (3.0.0) 1 | plg_system_logout (3.0.0) 1 | plg_system_skipto (4.0.0) 1 | plg_system_cache (3.0.0) 0 | plg_system_redirect (3.0.0) 1 | plg_system_remember (3.0.0) 1 | plg_system_languagefilter (3.0.0) 0 | plg_system_stats (3.5.0) 1 | plg_system_httpheaders (4.0.0) 1 | plg_system_actionlogs (3.9.0) 0 | plg_system_accessibility (4.0.0) 1 | plg_system_logrotation (3.9.0) 1 | plg_system_sessiongc (3.8.6) 1 | plg_system_sef (3.0.0) 1 | plg_system_webauthn (4.0.0) 1 | plg_system_languagecode (3.0.0) 0 | plg_system_fields (3.7.0) 1 | plg_system_updatenotification (3.5.0) 1 | plg_captcha_recaptcha (3.4.0) 1 | plg_captcha_recaptcha_invisible (3.8) 1 | plg_actionlog_joomla (3.9.0) 1 | plg_quickicon_privacycheck (3.9.0) 1 | plg_quickicon_downloadkey (4.0.0) 1 | plg_quickicon_joomlaupdate (3.0.0) 1 | plg_quickicon_extensionupdate (3.0.0) 1 | plg_quickicon_overridecheck (4.0.0) 1 | plg_quickicon_phpversioncheck (3.7.0) 1 | plg_editors-xtd_module (3.5.0) 1 | plg_editors-xtd_image (3.0.0) 1 | plg_editors-xtd_menu (3.7.0) 1 | plg_editors-xtd_pagebreak (3.0.0) 1 | plg_editors-xtd_article (3.0.0) 1 | plg_editors-xtd_readmore (3.0.0) 1 | plg_editors-xtd_fields (3.7.0) 1 | plg_filesystem_local (4.0.0) 1 | plg_behaviour_versionable (4.0.0) 1 | plg_behaviour_taggable (4.0.0) 1 | plg_content_joomla (3.0.0) 1 | plg_content_emailcloak (3.0.0) 1 | plg_content_loadmodule (3.0.0) 1 | plg_content_finder (3.0.0) 0 | plg_content_confirmconsent (3.9.0) 0 | plg_content_pagebreak (3.0.0) 1 | plg_content_vote (3.0.0) 1 | plg_content_pagenavigation (3.0.0) 1 | plg_content_fields (3.7.0) 1 | plg_media-action_rotate (4.0.0) 1 | plg_media-action_crop (4.0.0) 1 | plg_media-action_resize (4.0.0) 1 | plg_sampledata_multilang (4.0.0) 1 | plg_fields_radio (3.7.0) 1 | plg_fields_list (3.7.0) 1 | plg_fields_media (3.7.0) 1 | plg_fields_usergrouplist (3.7.0) 1 | plg_fields_editor (3.7.0) 1 | plg_fields_user (3.7.0) 1 | plg_fields_subform (4.0.0) 1 | plg_fields_color (3.7.0) 1 | plg_fields_imagelist (3.7.0) 1 | plg_fields_calendar (3.7.0) 1 | plg_fields_textarea (3.7.0) 1 | plg_fields_url (3.7.0) 1 | plg_fields_text (3.7.0) 1 | plg_fields_checkboxes (3.7.0) 1 | plg_fields_sql (3.7.0) 1 | plg_fields_integer (3.7.0) 1 |
3rd Party:: Installer - YOOtheme (1.0.3) 1 | PLG_INSTALLER_COOKIESPOLICYNOTIFICA (4.0.8) ? | plg_editors_tinymce (5.8.2) 1 | plg_editors_codemirror (5.62.2) 0 | PLG_AJAX_WEB357FRAMEWORK (1.8.4) 1 | PLG_AJAX_COOKIESPOLICYNOTIFICATIONB (4.0.8) ? | PLG_SYSTEM_WEB357FRAMEWORK (1.8.4) 1 | System - YOOtheme Framework (2.6.0-beta.2) 1 | plg_system_jcemediabox (2.1.1) 1 | PLG_SYSTEM_COOKIESPOLICYNOTIFICATIO (4.0.8) ? | Fields - YOOtheme Location (2.6.0-beta.2) 1 |

Templates :: Site :: yootheme (2.6.0-beta.2) 1 | cassiopeia (1.0) 1 |
Templates :: Admin :: atum (1.0) 1 |

[StiffeL]

Can confirm from previous posts that the Swedish language pack is the problem. Once I rolled back, uninstalled it the upgrade went just fine.