This thread is resolved. Here is a description of the problem and solution.
Problem: After installing the WPML Multilingual CMS plugin, the site admin panel stops working and displays a critical error. The error message indicates that the class 'DOMDocument' is not found, which is part of a PHP extension that may not be enabled on the server. Solution: If you're experiencing a critical error after installing the WPML Multilingual CMS plugin and the error message mentions that the 'DOMDocument' class is not found, we recommend checking if the PHP XML extension is enabled on your server, as the 'DOMDocument' class is part of this extension. A similar issue was resolved by another client, and you can follow the steps provided in this thread: https://wpml.org/forums/topic/fatal-error-when-trying-to-access-installed-plugin/#post-12615561.
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After installing the "sitepress multilingual cms" plugin, the site admin panel stops working and gives a critical error.
Until today the site was working fine. Today I received an email with the following content:
" Howdy!
WordPress has a built-in feature that detects when a plugin or theme causes a fatal error on your site, and notifies you with this automated email.
In this case, WordPress caught an error with one of your plugins, WPML Multilingual CMS.
When seeking help with this issue, you may be asked for some of the following information:
WordPress version 6.4.2
Active theme: Hendon Child (version 1.0.0)
Current plugin: WPML Multilingual CMS (version 4.6.8)
PHP version 8.0.30
Error Details
=============
An error of type E_ERROR was caused in line 67 of the file /home5/sqinfosq/public_html/wp-content/plugins/sitepress-multilingual-cms/classes/xml-config/class-wpml-xml-config-validate.php. Error message: Uncaught Error: Class "DOMDocument" not found in /home5/sqinfosq/public_html/wp-content/plugins/sitepress-multilingual-cms/classes/xml-config/class-wpml-xml-config-validate.php:67
Stack trace:
#0 /home5/sqinfosq/public_html/wp-content/plugins/sitepress-multilingual-cms/classes/xml-config/class-wpml-xml-config-validate.php(51): WPML_XML_Config_Validate->get_xml()
#1 /home5/sqinfosq/public_html/wp-content/plugins/sitepress-multilingual-cms/classes/xml-config/class-wpml-xml-config-validate.php(34): WPML_XML_Config_Validate->from_string()
#2 /home5/sqinfosq/public_html/wp-content/plugins/sitepress-multilingual-cms/classes/xml-config/read/class-wpml-xml-config-read-file.php(18): WPML_XML_Config_Validate->from_file()
#3 /home5/sqinfosq/public_html/wp-content/plugins/sitepress-multilingual-cms/classes/xml-config/class-wpml-config.php(306): WPML_XML_Config_Read_File->get()
#4 /home5/sqinfosq/public_html/wp-content/plugins/sitepress-multilingual-cms/classes/xml-config/class-wpml-config.php(43): WPML_Config::parse_wpml_config_files()
#5 /home5/sqinfosq/public_html/wp-content/plugins/sitepress-multilingual-cms/classes/xml-config/class-wpml-config.php(34): WPML_Config::load_config_run()
#6 /home5/sqinfosq/public_html/wp-content/plugins/sitepress-multilingual-cms/inc/translation-management/translation-management.class.php(258): WPML_Config::load_config()
#7 /home5/sqinfosq/public_html/wp-includes/class-wp-hook.php(324): TranslationManagement->init()
#8 /home5/sqinfosq/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters()
#9 /home5/sqinfosq/public_html/wp-includes/plugin.php(517): WP_Hook->do_action()
#10 /home5/sqinfosq/public_html/wp-settings.php(643): do_action()
#11 /home5/sqinfosq/public_html/wp-config.php(95): require_once('/home5/sqinfosq...')
#12 /home5/sqinfosq/public_html/wp-load.php(50): require_once('/home5/sqinfosq...')
#13 /home5/sqinfosq/public_html/wp-admin/admin.php(34): require_once('/home5/sqinfosq...')
#14 /home5/sqinfosq/public_html/wp-admin/themes.php(10): require_once('/home5/sqinfosq...')
#15 {main}
thrown "
After deactivating the plugin, the admin panel started working.