Errors in plugins make a 500 server error with no error message on screen or in log

Solution:1

WordPress disables error reporting by default. You can enable wordpress debug mode by setting to true the WP_DEBUG constant in wp-config.php.

Here you can find more detailated instruction: https://codex.wordpress.org/Debugging_in_WordPress

Solution:2

The problem was caused by a plugin “WordPress CSV Importer”. I do not know why but removing it made debugging work again.