This is an issue related to the browser cache and the site entry point. You need to log in only from the POK homepage (https://www.pok.polimi.it/), after clearing your browser cache first (you can do this quickly by pressing ‘Ctrl + F5’) or by opening your browser in incognito mode.

If you can't access POK with your SPID or CIE credentials, you can create a Politecnico Account by following these instructions.