You posted a screenshot of Chrome with 500 errors failing to load components/structure/index. However, there is no such string in any of these CSS files.
Did you obfuscate your CSS by adding url(http://website/subweb)? If not, then maybe this is 404.
Open the Chrome Network Tab like you did before and find the file with components/structure/index in it. If you see an error in the Chrome Network Tab then you should be able to find the source of the error by clicking around in the interface.
deleting CSS files in /wp-content/uploads/sites/5/css (assumes FTP access)
theme options > adding a language save (removed later)
clear browser cache
book info > verify no issues
note 1 - omitting step 2 only restored editor.css
note 2 - css files return to default 644 permissions (instead of previous 775)
note 3 - opening book info takes a moment to load Copyright Notice and Long Description
error no longer displays
Thank you, there appears to be several solutions in this discussion that may solve the css issue.