I’ve stumbled across the exact same problem this morning.
I have a repeater field set to appear using conditional logic (when a checkbox is checked) and that works fine. I’m now trying to add some radio buttons which appear on the same logic but it won’t let me select anything – it has an ‘add rules’ button but that does nothing, same as you’ve reported.
The only difference is that I don’t see the PHP error you’re seeing when you click update – mine just refreshes with no error.
This is with ACF 5.0.1.
