my server some web 500 internal Server Error but not show error log

my server some web  500 internal Server Error but not show error log i don't know why

i use Joomla 3.5.1

joomla 2.5 work

apache error log

SoftException in Application.cpp:567: Directory "/home/mbanida/conference/2017" is writeable by group
 Premature end of script headers: index.php

use
find . -type d -exec chmod -R 755 {} \;

find . -type f -exec chmod -R 644 {} \;

work for fix

Maybe you're looking in the wrong log try /usr/local/apache/logs