5 am this morning I woke, because a thing was beeping somewhere. The sound was quite annoying, so I had to investigate. My NAS-box chose to go into error state during regular backup schedule and inform me with this audible alarm, that hard drive was full. Bullshit! I said.
In the front-panel there was a flashing message "Error message volume full". However, the hard drive was far from being full. With the text, I googled into this article about QNAP HDD full. The clue about Network Recycle Bin was helpful. The system information really said, that all of the available hard drive capacity was taken. Majority of the stuff I had there was files in the Network Recycle Bin.
The fix was easy: I reduced the file retention time into half of the previous and clicked "Empty". The box chewed the drives for a couple of minutes and new stats were much better. Also the annoying beep ended.
Falling back to sleep at 5:30 was another thing. No could do. Unfortunately.