When I hit control-z to undo inside the actionlist editor, it seemed to have corrupted the action list. It just shows the purple single line and two white dots. I've restarted a few times to no avail. It looks like I may have lost that particular action list to a void in the universe.
Comments
It sounds like your actionlist may be corrupted - not just the action itself. Best to transfer the actions to a new object - use the window to select all, copy, then paste in a new Cutscene or whichever ActionList type you're working with.