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This thread is resolved. Here is a description of the problem and solution.
When I activate WPML plugin, I am getting this error:
Fatal error: method_exists(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "unknown" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide an autoloader to load the class definition in /var/www/vhosts/infomerciale.info/zespa.infomerciale.info/wp-content/plugins/sitepress-multilingual-cms1/classes/language-switcher/class-wpml-ls-templates.php on line 304
We have another client who had the similar issue and was also able to solve it, this may help in your case: https://wpml.org/forums/topic/cant-activate-wpml-3/#post-1560411
- To solve this issue you need to do a migration without ManageWP clone since the issue is happening due to migration.
- OR You can use some other plugins to migrate site such as Duplicator or manually migrate the site: https://wordpress.org/plugins/duplicator/