PuTTY in your hands: SSH client gets patched after RSA key exchange memory vuln spotted: That really is a 'game over' level vulnerability for a secure network protocol: a MITM attacker could bypass the SSH host key system completely.
The problem is that we use the tortoisesvn builds of putty so that we get a GUI for password and key confirmation. Since we can't wait for them to make a new release, r22126 switches to paramiko as default on win32 (now identical to all the other platforms).
To go back to the previous behaviour, and ignoring the serious security vulnerability (...), use xpra --ssh="C:\Program Files\Xpra\Plink.exe -ssh -noagent".
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2222