Good evening Silvia and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the 404 error message you are getting.
I would love to help you with this topic.
In this specific case, http://www.maremma24.it/events/ is not a category page, but rather the calendar’s main URL.
You should be able to get that back by going to Events -> Settings -> General.
Once there, look for the Events URL slug , adapt it to your liking and re-save this setting page.
You also might need to re-save your permalinks in the process.
Also, while I am at it, please know that for now, we officially do not support Polylang, but rather WPML (which we are in the process of supporting even better).
That being said, many of our customers have been able to get Polylang to work great with our plugin.
Let me know if that helps.
Have a great day!
Geoff B.