Thanks for writing in! Please make sure you upload x.zip file. This issue usually happens when you try to upload x-package.zip file.
Please unzip x-package.zip file and find and upload x.zip file to your site. Also make sure to delete the current theme folder which you have incorrectly installed.
How can I bee such stupid… I’ve been working with your theme before which included an installation. I blame on the first day of working on the site after some Christmas vacation. Sorry and thanks, Fixed and done!