If you do have more than one network interface in the Windows machine you are trying to run WinBox on, try disabling all other interfaces apart from the one where your RouterOS machine is connected.
I usually have to disable all interfaces except the one connected to the router when connecting via MAC address, instead of IP. however usually it gives a .dll error, but this might be similar.
Hope that helps
I always get in BUT I have the same problem sometimes. I went round and round the other day trying to connect to a new rb112 and then finnally gave up and winboxed in from another PC with out a problem.
It's a pain sometimes but I still like the stuff from Mikrotik:o)