When you connect to Linux-based servers you should connect as root (Login='root').
If so, could you please check connection with your Linux server using some SSH-client?
This might sound stupid and I am sure you have already done this, but try rebooting both servers and then run the PMM. When I was getting errors like that from PMM I would reboot both servers and the message would go a way. Of course my problem had to do with the Linux server kept locking up for no reason and that is why I had to move to a new server, but that is another story in its self.
I just thought of something after my last post. What is the firewall set to in Plesk on the Windows server, make sure SSH port 22 is open on that server as well, through its firewall, I think I had the same error until I did that.