Facebook messenger checkbox plugin is hidden (already checked other answers)

I have the same problem like asked here: Facebook messenger checkbox plugin is hidden

I tried the implementation of the Facebook Checkbox Plugin according to Facebooks documentation but it won't show on my VM in my Company.

The Facebook SDK Implementation works fine, I tried the "Share" and "Like"-Button, which show up correct.

I have a Facbook Page which has public access and a Messenger APP in DEV-mode. The "Send-API" from Facebook works correct too: I am able to send message to myself as administrator from my Webpage on my development VM.

Following Steps are done: use a random user_ref on every request, set my page on the whitelist, use "https", visit my page FB-loggedIN and FB-loggedOut

But the result ends in Markup and Log-Infos like this screen: See FF DeveloperTools Markup Screenshot for current state

According to this I visited a page given as origin where the Plugin should work (see first link in my question) - but there the plugins' HTML is the same like mine.

So is there a general problem on Facebooks' site or is there quite more necessary, to get this plugin running? Could it be a problem with self-signed certificate on my VM?

At the moment I don't want to notify or login any user. All I want is that the checkbox is shown.

Pherhaps anyone has an idea or the same experiences :)