Wild guess vCenter 51.?
In C:\Program files\Vmware\Infrastructure\SSOServer\logs should be an imsTrace.log.
Try logging in with typing username and password and then have a look into that log. If there is a java backtrace telling that it cannot establish a managed connection to your domain controller you will need to fix your identity source configuration.
You can also try to use the web client to login, as the error message you are getting there tends to be more specific and should pop up the according kb articles when you search for it.