No connection through FTPS (990)

NetDrive version: 3
Windows/Mac Version: 10

Dear Netdrive3 people,

I tried to connect to a server which runs on windows server 2016 standard.
This server can be accessed using FTPS in passive mode and SSL/TLS.
However, netdrive responds:

Please check the connection information.

Other FTP programs connect well using the credentials provided. Any ideas?
Thanks in advance,

I found out that the server supports SSL under port 21. That solved it: I now use port 21 in stead of 990.