Viewer 3.1 cannot authenticate to UltraVNC

Using viewer on Mac 10.6.8. Can connect to linux and UltraVNC servers on XP. Used to be able to connect to UltraVNC server running on Win7 x64. Cannot connect now. Keep getting “Unable To Authenticate”.

I can however connect to the Win7 UltraVNC server with several other clients. So it seems that the server is fine and the password is good.

Any ideas how to troubleshoot this?