Hi Heike,
Thanks for writing in! If you have idea about the error or look closely then you will see that those are 404 errors. That means either you have removed the files from the theme (that’s unusual and I believe you didn’t) or somehow your server is not delivering them for some reason like permission issue, network issue etc. Note that X is developed over the years and there is no such silly issues with the theme. Some of your errors related to WordPress core files; that doesn’t mean there is really something wrong with WordPress rather it’s related to your site/server and most of them could be ignored.
Hope this makes sense.