Menu goes crazy after any changes to it.

Hi guys. I am having a problem regarding my top menu (https://www.spszansa.pl). It’s not plugin related or WP version related (I ruled that out). Until yesterday I didn’t have any problems with it. What happens is that after I change the menu in any way (from the administrative panel), i.e. add a page or remove it, after saving it menu goes crazy. It shows some old tabs, about 30 of them, and cannot be modified in any way. Photo here: https://postimg.org/image/klktg7u0x/

I was wondering if anyone could help. Thank you.