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This thread is resolved. Here is a description of the problem and solution.
The following error showed in the debug log.
PHP Warning: array_key_exists() expects parameter 2 to be array, null given in /wp-content/plugins/sitepress-multilingual-cms/classes/action-filter-loader/wp-blogs.php on line 3
It is not certain what caused this error to show. I suspected that there was a corrupted file in WPML installation. Deactivating and deleting the WPML and then installing a fresh version of it probably solved the issue.
1. Take a full backup of your site in case something goes wrong.
2. Deactivate WPML Multilingual CMS plugin.
3. Delete the plugin.
4. Download the plugin from here https://wpml.org/account/downloads/
5. Upload it to the plugins section on your site and install and activate it.