There are multiple causes for this, so it's hard to say. First, make sure that you haven't installed the new "always-on" profiling module into IIS. This will prevent you from doing regular profiling of IIS, so you will need to remove the profiler module from the IIS configuration before you can use ANTS the regular way.
This can also happen when the profiler Core DLL is not registered. In some extreme cases, it indicates a failure in ProfilerCore.dll's initialize method.