[Resolved] Fatal error on accessing wp-admin

I am trying to: access my login page via wp-admin, but since 1-8-2023 this is giving intermittend critical errors. Login by going to wp-login.php does work (most of the time). Once logged in however the frontpage (dutchmuseumgiftshop.nl) also starts giving critical errors.

Link to a page where the issue can be seen: dutchmuseumgiftshop.nl/wp-admin

I expected to see: a login page

Instead, I got: Sometimes I get the login page, many times I get a critical error. Sometimes the critical error appears immediately on accessing wp-admin, sometimes it only occurs after supplying login credentials.

The server logging shows the following error.

AH01071: Got error 'PHP message: PHP Fatal error: Uncaught TypeError: Illegal offset type in isset or empty in /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/sitepress-multilingual-cms/sitepress.class.php:3937\nStack trace:\n#0 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/sitepress-multilingual-cms/classes/translations/class-wpml-post-element.php(54): SitePress->is_display_as_translated_post_type()\n#1 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/sitepress-multilingual-cms/classes/url-handling/wpml-url-filters.class.php(445): WPML_Post_Element->is_display_as_translated()\n#2 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/sitepress-multilingual-cms/classes/url-handling/wpml-url-filters.class.php(238): WPML_URL_Filters->is_display_as_translated_mode()\n#3 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-includes/class-wp-hook.php(310): WPML_URL_Filters->permalink_filter()\n#4 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-includes/plugin.php(2...', referer: hidden link

Also received the following Error by mail from the wordpress site:

Error Details
=============
An error of type E_ERROR was caused in line 3937 of the file /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/sitepress-multilingual-cms/sitepress.class.php. Error message: Uncaught TypeError: Illegal offset type in isset or empty in /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/sitepress-multilingual-cms/sitepress.class.php:3937
Stack trace:
#0 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/sitepress-multilingual-cms/classes/translations/class-wpml-post-element.php(54): SitePress->is_display_as_translated_post_type()
#1 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/sitepress-multilingual-cms/classes/url-handling/wpml-url-filters.class.php(445): WPML_Post_Element->is_display_as_translated()
#2 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/sitepress-multilingual-cms/classes/url-handling/wpml-url-filters.class.php(238): WPML_URL_Filters->is_display_as_translated_mode()
#3 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-includes/class-wp-hook.php(310): WPML_URL_Filters->permalink_filter()
#4 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-includes/plugin.php(205): WP_Hook->apply_filters()
#5 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-includes/link-template.php(371): apply_filters()
#6 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-includes/link-template.php(201): get_post_permalink()
#7 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/google-site-kit/includes/Core/Assets/Assets.php(1099): get_permalink()
#8 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/google-site-kit/includes/Core/Assets/Assets.php(728): Google\Site_Kit\Core\Assets\Assets->get_product_base_paths()
#9 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/google-site-kit/includes/Core/Assets/Assets.php(357): Google\Site_Kit\Core\Assets\Assets->get_inline_base_data()
#10 [internal function]: Google\Site_Kit\Core\Assets\Assets->Google\Site_Kit\Core\Assets\{closure}()
#11 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/google-site-kit/includes/Core/Assets/Script_Data.php(51): call_user_func()
#12 [internal function]: Google\Site_Kit\Core\Assets\Script_Data->Google\Site_Kit\Core\Assets\{closure}()
#13 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/google-site-kit/includes/Core/Assets/Asset.php(129): call_user_func()
#14 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/google-site-kit/includes/Core/Assets/Assets.php(1017): Google\Site_Kit\Core\Assets\Asset->before_print()
#15 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/google-site-kit/includes/Core/Assets/Assets.php(1026): Google\Site_Kit\Core\Assets\Assets->run_before_print_callbacks()
#16 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/google-site-kit/includes/Core/Assets/Assets.php(1026): Google\Site_Kit\Core\Assets\Assets->run_before_print_callbacks()
#17 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/google-site-kit/includes/Core/Assets/Assets.php(1026): Google\Site_Kit\Core\Assets\Assets->run_before_print_callbacks()
#18 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-content/plugins/google-site-kit/includes/Core/Assets/Assets.php(156): Google\Site_Kit\Core\Assets\Assets->run_before_print_callbacks()
#19 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-includes/class-wp-hook.php(308): Google\Site_Kit\Core\Assets\Assets->Google\Site_Kit\Core\Assets\{closure}()
#20 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters()
#21 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-includes/plugin.php(517): WP_Hook->do_action()
#22 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-admin/admin-header.php(146): do_action()
#23 /var/www/vhosts/dutchmuseumgiftshop.nl/httpdocs/wp-admin/index.php(137): require_once('...')
#24 {main}
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