I moved WordPress from one server to another and imported everything with the standard WordPress importer. As there are a lot of posts and about 10000 images and videos, I uploaded them with FTP.
Now all the images inside my posts are correctly linked (Maybe just because I used "Velvet Blues Update URLs"), only my featured images are gone. The media library is empty, and the featured images are empty, too (Not even a link that points to nowhere).
I tried all kinds of export/import-plugins and a plugin named "Add from server". But none of them worked.