Solution:1
I wanted to add this as comment, but unfortunately I don’t have enough points.
This can be because of some plugin messing with HTTP headers – have you installed some plugin/theme soon before the errors start showing?
Can you enable WP_DEBUG, to see if you can catch something there, as sometimes it is more useful than the server logs.
To do this, open
wp-config.php
located in the directory where WordPress is installed and then locate the line with the code
define('WP_DEBUG', false);
and change it to
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);
That will save the log in a file called debug.log and located in your wp-content
directory. It is important step to set it log to a file, if you don’t want to have errors thrown on the site.
You can also refer to this codex page.