The 251 is not a valid response to PWD command.
To force WinSCP to accept it, please configure this raw session settings:

https://winscp.net/eng/docs/ui_login_raw

This may be too specific to be reasonably fixable, but here's hoping:

I am trying to connect to an HP 1670G logic analyzer, an old piece of electronics test gear from the late 1990s. One of the ways to access it is via FTP.

When I try to FTP from a Linux shell, it works fine. However, when I try to connect via WinSCP, it seems to have a problem with the unit's response to the PWD command. There's nothing I can do about how the unit responds - it's long out of production, and HP certainly won't issue patches for a 30+ year old device. Still, it'd be nice to be able to access it with a graphical FTP client from my Windows PC.

Is there any sort of fix for this?

I'm using the latest version of WinSCP, and a log is attached. I've also attached a screenshot of an ftp session from linux.

Thank you.