azavalin on "[Plugin: Polylang] Fatal error after update to 1.7.2. , but not for Firefox"

Hello, after the recent update, for several browsers but not for the Firefox a Fatal error appears:
Fatal error: Cannot use object of type __PHP_Incomplete_Class as array in /home/content/p3pnexwpnas05_data01/25/2261625/html/wp-includes/functions.php on line 3185
or for another page:
Fatal error: PLL_Model::_languages_list(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "PLL_Language" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /home/content/p3pnexwpnas05_data01/25/2261625/html/wp-content/plugins/polylang/include/model.php on line 209

https://wordpress.org/plugins/polylang/