Please attach complete log files both from GUI and script.

Changed script to:

open -explicittls ftp://username:password@server:21

I get a reply that the Host does now exist!

The username, password, server and port connect with no problem with the GUI and FTP over explicit TLS.

Is some other command that is default in the Gui required?

Thanks

Use dash, not slash before explicittls.
See
https://winscp.net/eng/docs/scriptcommand_open

I can get into a server requiring FTP over explicit TLS with the GUI, but through a script I get a prompt for HOST.

Tried :
option batch abort
option confirm off
open /explicittls ftp://username:password@server:21

?