A database update was required after one of my recent WordPress upgrade, which turned to a 500 Internal Server Error. No matter how many times I tried, I just couldn’t pass the data upgrade screen to get into the WP-Admin panel.
Look further to the error log on the server, it’s the line below that triggered the 500 Internal Server error.
[Thu Mar 12 15:00:38 2015] [error] [client 18.104.22.168] Premature end of script headers: upgrade.php,
And adding the following line in the wp-config.php file magically fixed the issue.
Hope it helps if you encounter the similar error when upgrading WordPress and its database.