Error when using your own maintenance.php

I use wp-maintenance since quite some time now. I always had my own maintenance.php in my theme-folder which worked perfectly. With the latest update of wp-maintenance, this does not work anymore. The page is shown completely empty an a look at the page source reveals that the code stops right at the call of

get_header();

in my theme’s index.php.

Any ideas on that?

https://www.ads-software.com/plugins/wp-maintenance/