Labists ET4 printer nozzle hits bed after automatic levelling
Dirt. It was dirt. Most likely.

Apparently the machine establishes bed level with a sensor, and I might be wrong, but I think the ET4 monitors capacitance as a means to notice how far the nozzle is from the bed.
After all else failed I looked at the nozzle closely and noticed that the whole thing looked fuzzy. Looks like soot (from where though!?) had settled on the machine's sensitive components and de facto blind-folded its sensor.
A wipe and a levelling later, all was back to normal. Yay!