[Tue Dec 05 21:09:30.546838 2017] [:error] [pid 8486] [client 94.63.XXX.XXX:60633] SoftException in Application.cpp:388: UID of script “/home/ftpuser/public_html/index.php” is smaller than min_uid
[Tue Dec 05 21:09:30.546875 2017] [core:error] [pid 8486] [client 94.63.XXX.XXX:60633] End of script output before headers: index.php

Try to chown the files and directories to the proper apache / cpanel user.