we currently have this Problem with the missing YouTube Preview images and I can't log in on GitHub now.. so I write here..
Try to embed a YouTube video with the youtube or the video bbcode.. There seems to be something wrong with the video id. Maybe that is our problem ......
When I embed like this, everything works: [youtube]https://www.youtube.com/watch?v=KDhoM8fhQUc[/youtube]
My problem yesterday was, that I removed the m. (for mobile) from the URL and tried to embed with that address (without www.): [youtube]https://youtube.com/watch?v=KDhoM8fhQUc[/youtube]
And that doesn't work. Maybe that could be improved. But seems to be an other issue..
@noidea Initial feedback from Netcup was that they do not block anything themselves. A ping confirms this. They are currently checking whether there is another (network) problem. I have named 4 domains that are affected at Netcup.