A few days before I planned to harvest I had noticed that there was some nektar / honey in the bottom collecting tube.
I am not sure how it got in there, but perhaps some of the cells weren’t properly closed by the bees before they started putting nectar in or perhaps a part of the frame shifted during an inspection, letting nektar flow down into the collecting tube.
Because the bees had closed the drip hole with propolis, it didn’t flow back into the hive for them to re-collect and process it properly.
I think it might already have started to ferment, so it didn’t taste good and I had to clean it before I was able to harvest proper honey.
So my tip is: Use the flow tubes to re-open the drip holes every now and then. This way - if any nektar drips down into the collecting tube - it can drip back into the brood box for them to re-process it.