cannot remove prototype link

Select those line not a ending node and press DEL

Oh wait – now that I can see the video (I overlooked it earlier...) I guess you're suffering from old behavior. Unlinking can be a bit difficult, since it's possible to have multiple links from one item (I know, yours is the other way around, but bear with me). Use the Delete or Backspace key to remove it.

The idea behind it, that disconnecting the handle would remove all connections and triggers (a new feature). So you have to select the line or that moving dot, and delete it by using a key. Yeah, it's very silly, but the developers thought it would be nice to throw us off a bit, here. 😉

That's odd – maybe some glitch...

Try using the Properties panel on the right to change the Destination manually with the menu to something else and back again, in order to make the handle behave properly.