Thanks for your feedback.

Thanks, it works. You were correct, window is displaying windows system language string.

I've resent the email.

Dear Martin, I hope, I understood you correctly, but I didn't receive any nightly build, could you please resend it to me in order to test it?

Only after I've responded, I've realized that it's probably about Windows localized messages.
I have added this issue to the tracker:
Issue 2197 – Localized HTTP connection error messages are incorrectly decoded
I'm sending a development version of WinSCP to your email.

Thanks for your report.
What exactly does "the codepage win1251 for non-utf symbols" mean?
I've assumed that you mean the "Language for non-Unicode programs" (see https://winscp.net/eng/docs/faq_utf8#ansi).
But I've tried to set it to "Russian (Russia)", and do not see the problem. I get correct

Host "winscp.net" does not exist.
Could not resolve hostname `winscp.net': No such host is known.

On the codepage win1251 for non-utf symbols I'm getting this window when trying to update WinSCP (6.1) without internet. This codepage bug affects all languages. There could be more of them, but I found only one.