To work around this issue for the Beta, do two things.  First, add the following to your ~/Web.config if it is not already there:

<appSettings>
        <add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />
      </appSettings>

Second, add this as the first line within your Task-returning action method:

await Task.Yield();

The second part (the 'await Task.Yield();') will not be necessary once this bug is fixed post-Beta.

Hope this helps!