Black rectangle instead of a thumbnail

Have you clicked the “Update Posts” button below the “Save Changes” button already? And uncheck the “Only load CSS/JS when needed” setting. Does that change anything?

Yes, updated posts and have “only load CSS” unchecked – it doesn’t work.
Would you like to check how it looks on the page itself?
Thanks for the quick response.

Can you send me a link to your site so that I can see the issue myself?

Of course,

https://kulturawplot.pl/2015/04/22/prima-aprilis-najlepszy-zart/

Halfway down the post.

You must activate the plugin so that I can see the problem…
I guess there’s a conflict with your theme.

It is activated. Trust me on that ??
What kind of conflict it might be?

Ah, I see. I expected the video to be “black”. But for me, it is showing up. (In the wrong way, however.)

You are using several plugins. Maybe Better WordPress Minify causes the scripts to load not as expected.
Please deactivate all plugins (except for Lazy Load for Videos) to find a plugin that might cause the conflict. When every other plugins are deactivated, Lazy Load for Videos should work. Or there is another problem.

Bingo my friend!

I’ve excluded Lazy Load from Minify and it works perfectly!

Thank you very much! Excellent support!

OK, I see there is still a problem with Firefox.
On Chrome it works fine now, but Firefox stays the same – black rectangle.

One more issue:

Some videos now load correctly, and some links are overtaken by WordPress default video embed, and there is no way to stop that.

Here’s an example. The plugin is turned on.

Here it works:

https://kulturawplot.pl/2013/09/06/youtube-cale-filmy-online/

Here it doesn’t:

https://kulturawplot.pl/2015/02/02/dokumenty-youtube-vice/

Please go to the article where the videos are not converted as expected. Update the article. Does that change anything?