Hi,
I got an error message when trying to connect (FTP) to a host with a german "Umlaut" in it. These special characters are ä ö ü ß.
The ACE version of that hostname works instead.

Testet with latest release version 5.5.1 (Build 3970)

Error message:
Can't resolve host name "www.öko.de" - Connection failed.

Example:
IDN: www.öko.de
ACE: www.xn--ko-eka.de

Thanks for your post.
This has been on PuTTY wishlist for a while:
https://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/idn.html

Hi,

I have testet it on a windows 7 prof. 64bit german.

Other FTP software is doing right. Total Commander for example.

IDN: www.müller.de
ACE: www.xn--mller-kva.de

Details: https://www.denic.de/service/tools/idn-web-converter/