Sudden underextrusion on Ender3
A close inspection of what happened when printing the first layer resulted in this:


The missing steps on the new print came from the nozzle scraping too close to the print surface, which lead to no first layer
Readjusting the Z-axis end stop, which had moved down, resulted in no more lost steps, but the print not sticking for the first test.
Releveling the bed and making sure the bar was parallel to the bed resulted in a perfect first layer.
Lost steps and under-extrusion could not be replicated after 48 hours of rest for the printer.


I have no idea why the print had failed due to under-extrusion during the print, but apparently, my immediate tests were flawed enough to not detect the first layer height resulting in getting almost no extrusion. This I mistook for massive under-extrusion, making me believe something else was at fault.