I ran into this error: “Unable to locate WordPress Content directory (wp-content).” after upgrading the Linux distro on my server. Nearly every solution I found was to add a custom block of code to wp-config.php that would no longer require you to give the FTP credentials. I found this unappealing because I have multiple WordPress installations and I didn’t want to hack each one.
Instead I continued banging my head against the problem. WordPress and the permissions hadn’t changed so it had to be something in the environment. The two prime suspects were PHP and vsftpd, and vsftpd was it. SSL was enabled but I had to disable require_ssl_reuse in /etc/vsftpd.conf. I learned this by watching the log with debug_ssl and log_ftp_protocol enabled during a plugin upgrade attempt.
All it took was a quick restart of vsftpd and automatic plugin upgrades are working again with FTPS.