Hi, having the same issue:
Jul 25 15:45:22 hostname proftpd[2961]: 0.0.0.0 (x.x.x.x[x.x.x.x]) - USER ftpuser: Login successful.
Jul 25 15:45:22 hostname proftpd: pam_unix(proftpd:session): session closed for user ftpuser
Jul 25 15:45:22 hostname proftpd: pam_systemd(proftpd:session): Failed...