I was having the same problems trying to upgrade the WordPress version or plugins and was getting changing errors. After changing file permissions, manually deleting my upgrade folder, increasing the amount of PHP memory that could be used…I finally found out that the problem was with the ftp server that my host uses (well, the problem actually involved a WordPress bug, but it was solved by asking my host to change my ftp server temporarily to proFTP until I finished my upgrades.)
I wrote a detailed blog about it: https://arcandangle.com/wordpress-upgrade-cannot-copy-ftp-permission-issue-in-layman-terms
Hope this helps!
