500 Internal Server Error after Register dialog, but with successful registration

Internal Server errors are usually thrown when there’s an error somewhere in the code. You did the right thing searching the logs, but depending on your hosting configuration not all errors can be written there.

I like using WordPress’s own logging facility. Here’s what you need to do:

Stick the following lines in your wp-config.php file

define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);

Next time you get an Internal Server error, check your wp-content folder. You should have a file called debug.log that lists all the errors and warnings that your server has thrown. The last line usually tells you what the problem was (unless someone else has hit your site in the meantime)