I have an uncorrect css path in the front-end html code
Are the paths in the civicrm.settings.php files each pointing to the right place?

There's one for each of front and backend:
 - /components/com_civicrm/civicrm.settings.php
 - /administrator/com_civicrm/civicrm.settings.php

If you just copy and past the paths that are set those two files (e.g. after a server migration) then you might get the error you currently have. But the paths in one file need to point to front-end Civi, and in the other point to back-end Civi (ie with /administrator appended).