Booaahh, that is a nightmare :-)
So, FTPS the way VASmalltalk expects is FTP over TLS. In another FTP client, you select standard FTP protocol AND the option "Require implicit FTP over TLS" (e.g. in FileZilla). It needs as a counterpart a FTP server listening at port 990. The communication is secure from the beginning.
The alternative to it would be "Require explicit FTP over TLS"- here the initial connection is done over the old port 21 and then a protocol update is requested to get a secured connection.
I had to install proFTPd at a RaspberryPI4 to get support for the protocol and I had to read lots of documentation to get it to work. The best way is to use a "passive" client, you have to define a range of ports (max 256 here with a AVM Fritz home router) for the passive data connection and then of course you let the firewall know about that.
The IT-community seems not to be happy with "Require implicit FTP over TLS" standard because it defines two additional ports for its usage - considering the limited number of ports available, additional ports have to be defined in the firewall. It is considered as an old way of updating a protocol. The standard way today is the protocol update request (like http(s)).
So, nowe I am able to transfer automatically large files to a remote server (2MB text files) - but I still have trouble, that 30% of all transfers are aborted with a message like: "file transfer aborted. lost link to file server". This can be due to OpenSSL, proFTPd or whatever ...