sudo gedit /etc/proftpd/proftpd.conf
delete 3 lines :
TLSRSACertificateKeyFile /etc/gadmin-proftpd/certs/key.pem
TLSRSACertificateFile /etc/gadmin-proftpd/certs/cert.pem
TLSCACertificateFile /etc/gadmin-proftpd/certs/cacert.pem
save
restart gadmin !
this should solve the problem ! for me it dit !