After a closer look at my script, I found that I had two syntax errors on the one line so the script worked if there was a successful connection.
After making the corrections, I changed the desitnation IP address to produce a connection timeout. WinSCP now produces the correct errorlevel 1 on a failed connection.
Sorry to have bugged you.
Please attach a full session log file showing the problem (using the latest version of WinSCP).
To generate the session log file, use
/log=path_to_log_file command-line argument. Submit the log with your post as an attachment. Note that passwords and passphrases not stored in the log. You may want to remove other data you consider sensitive though, such as host names, IP addresses, account names or file names (unless they are relevant to the problem). If you do not want to post the log publicly, you can mark the attachment as private.