We've added some more debugging information since you reported this, so if you look in the Javascript Console again, you may see some a line beginning "Family Echo JavaScript error" that you can send to us.
Apart from that, the most helpful thing would be to isolate the problem by finding the specific situation where it happens. You could check if it only occurs with: (a) certain 'Tree view' settings, or (b) states of the new 'Friends/Pets' checkbox, or (c) entities with particular characteristics or relationships. Thanks for anything you can find.