I have added this issue to the tracker:
Issue 2388 – Dropbear SSH server on some devices rejects connection – possibly due to new KEX algorithms

Possibly the same issue:
SCP - Server unexpectly closed network connection

As I wrote above, WinSCP 6.5 supports all Key exchange algorithms that 6.3.7 did (and more).
The problem likely is that the device is not able to cope with the new algorithms that new version of WinSCP supports.
Did you try what I have suggested in my previous post?

Why is it considered a bug on the side of the device as WinSCP is the part that has been updated and therefore changed. The device was not updated. It is a router running the latest version supported of the merlin firmware.

It worked with V6.3.7 and fails with v6.5. What is the change in WinSCP that causes this failure? That's the part that I'm interested in.

Other SSH clients (Windows and bitvise) do work without problem.

I think I saw somebody else logging a bug with a similar issue. I did not try the beta versions.

WinSCP 6.5 supports all Key exchange algorithms that 6.3.7 did (and more).
So this looks like a bug in the device.
As ECDH KEX was used with 6.3.7, try moving that to the top of preference list:
https://winscp.net/eng/docs/ui_login_kex
Or moving new ML-KEM to the very bottom.

Hello Martin,

"Unsupported" is not the correct choice of word. I should have mentioned "Not Matching". The Router specifies in its logs: "No matching algo kex".

The only thing I can say is that it worked and "suddenly" stopped working. In terms of "changes", nothing changed on the router, and I always upgrade WinSCP to the latest available version using WINGET

• I was running WinSCP v6.5.2, which failed the SSH connection ONLY with this remote device
  
• Uninstalled WinSCP v6.5.2 and kept the WINSCP.INI
  

• Installed WinSCP v6.3.7 and reused the WINSCP.INI => SSH Connection to remote device WORKS!
  
• Upgraded to WinSCP v6.5.0 and reused the WINSCP.INI => SSH Connection to remote device FAILS!
  

Logs of both included

Thank you

So what has changed? Did you upgrade WinSCP? Did you upgrade the device?
Can you post WinSCP log? Did you try prioritizing another KEX in WinSCP? Why do you think this about "Unsupported" key exchange algorithm?

I use WinSCP to manage different devices at home though SSH using its interface.
Since some time when trying to connect to one of the devices I get the error: "Network error: Software caused connection abort". This started out of nothing
I'm currently using WinSCP v6.5.2
It took some time to understand this had to do with the Key Exchange between WinSCP and the device. In a previous version of WINSCP the SSH connection with the device succeeded.

This is what I tried:

• WinSCP and device --> FAILS
  
• PuTTY and device --> FAILS
  
• Plink and device --> FAILS (see attached file!)
  
• SSH Cli Client in Windows 11 and device --> SUCCEEDS
  
• Bitvise SSH Client in Windows 11 and device --> SUCCEEDS (see attached files!)
  

Looking at the log file from Plink, there appear to be mutually supported curves, but it still does not work, no matter what I do or try. With BITVISE it does succeed.

Is this a recent BUG in WinSCP? Is there anything I can do to try to see if it works?