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 as here:
WinSCP fails SSH connection due to "Unsupported" Key exchange algorithm
The same SSH server ("dropbear").

When attempting to open a tunnel-ed SCP protocol connection, starting with WinSCP 6.4.2 Beta, I immediately receive a "Server unexpectly closed network connection".

I can successfully connect with previous version of WinSCP, I can also connect on the latest version of WinSCP, if I change the Protocol to SFTP (but would highly prefer to have SCP working again).

Tested on the following Portable versions:

6.3.1 | works
6.3.7 | works
6.4.0 (beta) | works
6.4.1 (beta) | works
6.4.2 (beta) | does not work
6.4.3 | does not work
6.5.1 | does not work
6.5.2 | does not work

Attached some logs of the attempts on both 6.4.1 (working) and 6.4.2 (not working)

I even tried compiling the WinSCP 6.4.2 Beta version to attempt to add additional debugging messages, but I'll be damned if I am paying $2400 to simply compile WinSCP (...and yes, I even tried with the latest Community Version 23.0 and ran into compatibility issues while compiling)