Disallow cache folder to Googlebot: issues?

hmmm, I have no idea. but why do you need to do it?

Duplicate content, Googlebot crawls my FTP and sees the cache files as duplicates, then signals it in GWT – HTML Improvements section.

how does google reach /wp-content/cache/all?

No idea, It was not linked anywhere, so the only way is Google crawling my ftp folders

Don’t worry about this issue. SEO is not effected.