SSL breaks the plugin?

I used this plugin with success on my site. Then I added an SSL certificate as the only change, and after that the maintenance mode simply doesn’t show up. Even if it is enabled, all visitors can see the site when the maintenance mode is turned on. Tried re-saving settings and even removing / re-installing the plugin, but no change. It seems it can’t handle https://-pages.

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