Thanks, it didn't make any difference. I'll stick with 6.4.1 for now.

In 6.4.2, WinSCP added support for "post-quantum ML-KEM key encapsulation method". My guess is that the server chokes on encountering it in the list of supported KEXes. And instead of ignoring it, it crashes.
Try moving the method to the end:
https://winscp.net/eng/docs/ui_login_kex
But I do not really expect that to help.

Thanks for the reply. I should have worked through the versions to find where the problem started - I jumped versions so 'assumed' it was from 6.3.7.

I've now done that - the version where it stopped connecting is 6.4.2 Beta onwards. 6.4.1 Beta connects fine. Debug2 log attached.

Thanks for your report. So was 6.3.8 really the first version that does not work anymore? It does not look like there was any change in 6.3.8 that might have caused it. Can you post its log file?

I've been unable to connect to SSH-2.0-dropbear_2020.80 since WinSCP 6.3.7 – this is running on a modified Huawei router. Every version since 6.3.7 fails. I've tried modifying several settings in the connection config to get it working, but I'm not an SSH expert.

Attached are a log of a successful connection using 6.3.7, followed by a failed connection attempt using 6.5.6.

Hoping someone can point me in the right direction as I'd rather use the latest version of WinSCP.