Thumbnails broken for older posts.

After a wordpress move 1 year ago, my old posts don’t have any thumbnails attached to them anymore. Somehow the information of the thumbnails weren’t transferred. The photos in the posts are showing, however the thumbnails are broken in media gallery. This is a problem because I can’t get any plugin to create a featured image for the post. I have tried numerous.
I want to create an archive with thumbnails for my Recipes.

Is there any fix for that? If not,
I’m searching for a plugin that can take the first image URL in the post and create a featured image for the post, but don’t find anyone that work.