My issue is very closely related to Magmi for magento2 and this question here Fatal Error with Magmi for Magento2.
I've noticed when I enter my database credentials correctly in the magmi_engine.php file that the password I enter is stripped of capital letters.
If I manually amend in the magmi.ini file I get a 500 server error. Remove the capitals and the magmi page displays again. It is a requirement to have Capitals in the MySQL user password though so not sure how to resolve?