I am seeing the same issue with 5.1 update 2. I have no issues connecting to 5.1 update 1b vCenters but when I try to connect to ones I have updated to 5.1 update 2 it won't connect. I get the Loading Inventory and then get the message that it was disconnected.
You may try the new version (http://download3.vmware.com/software/vmw-tools/onyx/Onyx_2.2.5161.29309.zip) to see whether this works. I'm not sure, but this could be related to recent security changes to the default settings for the SSL connection.