This thread is resolved. Here is a description of the problem and solution.
Problem:Cant activate WPML
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 /www/htdocs/w0131108/staging6.ewerkstatt.com/wp-content/plugins/sitepress-multilingual-cms/classes/language-switcher/class-wpml-ls-templates.php on line 304
Solution:it is caused by the migration of your site using ManageWP clone.
If possible please delete your WPML and add-on plugins and re install them with a fresh version from the donwloads page.
Issue was resolved by creating a migration without ManageWP clone
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