Have a look at the oEmbed plugin https://github.com/ColdTrick/oembed/releases

It allows you users to simply paste in an url (like from YouTube, Imgur, Twitter, etc). If you don't wish to allow replacement of certain urls you can configure that on the admin side

Check the server error logs (Apache and/or PHP) to see what the problem is. The error message just says when the error has happened but not what is wrong. More info should be in the logs. Seach for "1541882771" as the timestamp should be included in the log entry. If the mog message doesn't help you, post it here. But maybe it already tells you what you might need to fix or do differently.

thanks, I fixed it. I had a mySQL Problem, Elgg is running now