Hello lucke-stz,
Thank you so much for your question. Actually this is not the theme issues. As per your errors, it seems that you missed some files or mistake the update query as well. Please follow the below steps:
1. Download the latest WordPress from http://www.wordpress.org
2. And replace the wp-includes & wp-admin folder with the latest WordPress.
3. Save the permalinks from the backend.
4. View front end.
Please let us know, we will available on live chat and you can ask there too.
Regards,
Acme Supports