TLDR: idle_session_timeout must not be zero.
Recently updated Debian server from Wheezy to Jessie left my SSL only vsftpd based FTP access crippled. Total Commander didn’t return any error, but log said:
SSL_read returned -1, SSL_get_error=1
OFFLINE2, error=0
SSL_read returned -1, SSL_get_error=5, ERR_get_error=0
Socket error: 10054