If you run into a PHP error, for example, an Internal Server Error, you’ll need to enable debugging on your WordPress site in order to identify the error in a meaningful way. To do so, edit your wp-config.php file as described here. Replace the part that normally sets WP_DEBUG to FALSE with
/**
* This will log all errors notices and warnings to a file called debug.log in
* wp-content (if Apache does not have write permission, you may need to create
* the file first and set the appropriate permissions (i.e. use 666) )
*/
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);
@ini_set('display_errors',0);
Now re-do whatever caused the error, and then check the debug.log file (again, don’t forget to make sure the server can write to it!)
The debug log normally gives a pretty clear explanation of the issue. The most common issues will be resolved by configuring your server properly, normally by increasing the memory_limit or max_execution time.