Update to wp newest version

Hello!

My current version of WP is the 3.9.1 and I’m not able automatically to upgrade it to any new version. The message says that

Could not copy file: wp-admin/css/wp-admin.min.css

I talked to my host and that has nothing to do with permissions and they say that is a wp issue…

Is there anyone experiencing the same issue? I have other sites hosted with bluehost or wit other free hosts and I have been able to automatically upgrade but not with that one whih is my main project :S

Thanks