TL;DR You likely hit a known
SSH.NET bug. Missing moduli file might have made it worse. Want to try .dll fix?
The asymmetry still bothers me, however I don’t know if computer speeds vary, e.g. perhaps B (on 32 bit OS) is slower than A. Nevertheless I’ll walk through the known speedup fix because it appears to match your use.
First look at
SSH.NET issue Key exchange is slow when size of group is more than 1024 bits #304
Software is same as yours. “WinSCP SSH server” reference links to winscp.net then the site you mentioned.
Exception is same as yours. Both look like SshOperationTimeoutException from Session.cs
Next look at
SSH.NET Session timeout caused by long time calculation on KeyExchangeDiffieHellman->PopulateClientExchangeValue #130
The enormous decimal number, when pasted into Linux dc to output as hex, looks a lot like the 8192-bit MODP Group which is also fixed into sshd to use when the moduli file is missing. Smaller values are also hardcoded, and the choice depends on the max requested. #304 shows 1024/min 8192/max requested.
Source of sshd shows this gets you the 8192 bit version because it only looks at max if moduli is missing.
Selection when moduli is there is complex. The man page says “sshd(8) then randomly selects a modulus from /etc/ssh/moduli that best meets the size requirement” so it’s not clear if it would have been any faster.
Next consider the missing moduli file, and what to do.
My Linux distro seems to have included an /etc/ssh/moduli file, but possibly your sshd did not bundle one. Possibly yours got lost somehow. You could compare your two systems, or check with your sshd provider.
Generating moduli is done with ssh-keygen, but see ssh-keygen -G modulus candidate generation failed
Bottom line here is that if you can figure out how to fix moduli easily, try it, otherwise maybe try a fixed .dll.
This is basically just backing up the one Duplicati ships, and replacing it with one downloaded in a zip file.
The challenges include making sure you replace the right one. Could you say what you installed (version
visible in Control Panel or Settings) and what version you’ve updated to (visible in Duplicati GUI in About)?