While you upgrade a plugin in WordPress, this message may bother you. It comes up when the
$wp_filesystem, which is an instance of
WP_Filesystem_XXX, can't create a temporary directory.
There are four
Direct, SSH2, FTPext, ftpsockets. And I think many of you are using
WP_Filesystem_FTPext to take advantage of FTP.
In my case, VSFTPD is the problem. Because Ubuntu turns off ftp's writable feature by default,
WP_Filesystem_FTPext can't make any file or directory. What I did to solve this is really simple: