At the end it is very easy to make it work 
As source use ftps://user:[email protected]:990/|auth=SSL/TLS
When creating certificate in FileZilla FTP server you ABSOLUTELY need to enter Common name (Server address) as shown on picture
![]()
Then you need to convert certificate from crt to pem format using openssl tool on LibreELEC box or somewhere else with command
openssl x509 -in certificate.crt -out certificate.pem
and copied to users' cacert.pem
cat certificate.pem >/storage/.config/cacert.pem
After that run openssl-config and restart kodi (systemctl restart kodi) or just reboot. And it should work like for me here.
Another option for certificates is to create them with openssl (as you probably know).
Update: I also removed few posts not adding any value to this discussion.