Thumbs are not displayed

Hello everybody,

I installed the plugin “Automatic Featured Images from YouTube / Vimeo” some time ago on two of my sites and used it successfully. There were never any problems. But if I create new posts today and insert YouTube URLs (I’ve tested it with different URLs per video), no thumb will appear in the post list. A gray substitute image will be displayed. Also in the post, the thumb is not displayed as a “post picture”. So there must be a problem when creating the image …

My sites:
https://www.massagepraxis-runte.de (latest post shows the problem)
https://www.praxis-runte.de (latest post shows the problem)

My pages were created on the themes “One Press Child Plus” and “Fluida”. Parallel to the Plugin Automatic Featured Images from YouTube / Vimeo I use the plugin “Smart YouTube”

Can someone help me solve the problem?

Thank you very much!

The page I need help with: [log in to see the link]