This error happens when called is no existed page which is 2 levels (no idea how to name it correctly) after existing link. Let me try to explain.
We can take for example restara section link from the demo site:
https://demo.sobi.pro/restaurant-guide
Now if we add some not existing sub page to this link it will return 404 error:
https://demo.sobi.pro/restaurant-guide/something
but if we add another level sub-page to this link:
https://demo.sobi.pro/restaurant-guide/something/somethingelse
there appear 500 error instead of 404.