I can't set register / login page.
My default register page is:
https://akademia.niemieckizpasja.pl/spolecznosc-akademii/?fcom_action=auth&form=register
and login page is:
https://akademia.niemieckizpasja.pl/spolecznosc-akademii/?fcom_action=auth
But they both don't work.
It displays fatal critical error.
Try modifying the permalinks in your Wordpress dashboard.
did you change the login and login url in FC settings?