If you do not allow your browser to store data in local storage, the node you were using to harvest won’t be recorded.
It is not possible for now to know the node you are harvesting on without storing it (like nano does) or query all 500+ nodes to check if you are harvesting on it (which is way too much queries).
Maybe this is why you saw “No free slots”, app has no node set so first of list is used and it has no slots.
It does not means that harvesting stopped. If you know the node you were using you can select it from the dropdown and it’ll restaure the status (will lock node dropdown and play button will become a stop).
If you use the correct node and it does not restaure automatically it means the node rebooted and that you need to choose a node with free slots, type you password and click on start button to start harvesting again.
Does it ? When dropdown is locked and button is a stop ? I am pretty sure I thought about that case