The problem with changing page errors

Please use monstaFTP and create/edit .htaccess file in htdocs folder of your sub/domain

edit if needed to look like this:

# error redirect
ErrorDocument 404

* of course also for other error codes if you have it
the error pages with the same names must exist
and if they are in a some subdirectory than edit the PATH (in example) to reflect the real location

make sure that the address of these pages is absolute (start with http/s)
and not relative

because if is relative…
when someone comes to the subfolder of the subfolder and ask something that does not exist or does not have permission it will appear infinityfree error pages not your custom