Maxime Culea on "[Plugin: Polylang] Fatal Error in admin option page"

What's the problem with PLL_Admin_Strings? I understood that your problem is coming from PLL_Settings::init() which does not exist.

If settings.php is present in polylang/admin, then this could be the source of the error. This file does not exist anymore since 1.8 (in fact it has been moved to polylang/settings).

If you mix two versions, then the autoload will require the wrong file, causing the fatal error that you are experiencing.

That's why it's mandatory to remove all files before updating. That's what WP does during a normal update. But if, like me, you sometime make manuals updates, you can experience such issues when the files are not all deleted before uploading the new version.