I could luckily witness that this happens when the nozzle temperature suffers from huge shifts. And when I say huge I mean that the temperature indication can jump from 260 to 180 degrees Celsius in a second and back to 275 etc. for several minutes, before it stabilizes back to more normal shifts of 2 to 5 degrees.
This, in some cases, produces what is shown in my first picture and in this second one herein attached. The print, luckily, could conclude successfully. In another case, the RAMbo reported def for both nozzle and bed temperatures and the print stopped midway.
As the temperature fluctuations are so relevant, and after having ensured that the power supply delivers what is needed I believe that the nozzle temperature probe is showing signs of tiredness, or maybe is the connection to the circuit board (HE280). I am excluding the heater cartridge as the temperature fluctuations wouldn't be so quick and sudden. This phenomenon does not occur at every print.