I believe the libssh 0.9.3 being the culprit. [...] So from my POV it would help if you could try using an older version of x2goclient (with an older libssh) to see if it works with that. OK, I tried this. X2Go switched (at least for the MS-Win build) from 0.7.4(!) to 0.9.3 on 2020-02-06, and indeed, the build from 2020-01-29 (the last one before the switch) works for me!
Could someone build x2goclient with libssh 0.9.4 so I could test that?
PS: I suspect this happens when the other side has a quite old openssh running, in my case it's OpenSSH 5.8, for you it's 5.9... So If you can upgrade the ssh server this might also solve the problem As stated in the original bug report: No, unfortunately, it's not the OpenSSH version that matters (I went into some lengths updating that, but it didn't help), but the base system version. I cannot easily update the login servers to NetBSD 8.