Own SSL certificate is marked as unknown while connecting via FTPS

[sparkxp]

Hi,

I'm bought a domain based SSL certificate (hosting.mydomain.com) for my Plesk panel. This is working fine.
Now I decided to use it for FTPS too. So I took a look in the /etc/proftpd.conf and saw that mod_tls is already implemented. The TLSRSACertificateFile points to /opt/psa/admin/conf/httpsd.pem, which is my working panel certificate.
But if I try to connect via FTPS to the FTP, I'm getting a message that the certificate is unknown. Within the message I can see that the correct certificate is used.
But why does it say its unknown??

I'm using WinSCP with the following login credentials.

Host: hosting.mydomain.com
Protocol: FTP
Port: 21
Encryption: tryed SSL and TLS explicit

My /etc/proftpd.conf

<IfModule mod_tls.c>
TLSEngine on
TLSRequired off
TLSLog /opt/psa/var/log/ftp_tls.log
TLSRSACertificateFile /opt/psa/admin/conf/httpsd.pem
TLSRSACertificateKeyFile /opt/psa/admin/conf/httpsd.pem
TLSVerifyClient off
TLSRenegotiate none
TLSOptions NoSessionReuseRequired
</IfModule>

WinSCP message: