Works fine now in v5.9.1 from Windows cmd and from PowerShell v5.
Thanks you for sorting this out.

This bug has been added to the tracker:
https://winscp.net/tracker/1447

I have run the debug version and sent the zipped log files attached to an e-mail to you.

Thanks for your report.
I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.

Tried running open command with -rawsettings utf=1. This had no effect.
Similar problem recently posted in another part of the forum: https://winscp.net/forum/viewtopic.php?t=23458

Have been successfully running three separate PowerShell ( v.5) script jobs, under Windows 10 Home, to call WinSCP (version 5.7.5) to use SFTP to upload, download, and synchronize files.
Downloaded and installed version 5.9 of WinSCP and now get above error from the three jobs.
Ran call to WinSCP.com from a Windows .bat file and still get the same error as when running from PowerShell. No WinSCP log is produced unfortunately.
Powershell code line:
winscp.com /script="$WinSCP_Dir\WinSCP-cmds.txt" /log="$winscp_log" /loglevel=1 -timeout=60 1>>$progress_log
WinSCP command script (passwords etc. altered but same character set used):
option batch abort
option confirm off
open sftp://u12345678-abc_test:3y49anPgr@abcd123456789.1and1-data.host
put -transfer=binary mytest.txt
close
exit