Why is ice sticky between −20 to −8 degrees Celsius but not below −30 °C?
I think I remember that episode. Basically, crystals of water in kerosene were not a problem during the whole flight (at minus thirty something) but crystals were too sticky when the temperature raised to -20°C and prevented the fuel to go to the engines just a few millimeters before entering the "warming cell" which was supposed to prevent any such blockage.

Crystals at low temperature (let's say -35°C) will be quite hard and quite big, even if some molecules of the fuel will be trapped in the crystals of ice (because there is much more fuel than water). If the temperature goes up to a certain level (let's say -20°C), crystals will break where there was molecules from the fuel. As a consequence, the big crystals will break into very small crystals. Ice crystals do not like their environment (i.e. the fuel) because they cannot find any stabilizing effect with it, but they love other ice crystals. Moreover, the overall surface greatly rises when the size of the crystals diminishes. As a consequence, they will try to stay together and form a "slush".

In that accident, it was not the only factor. The other two problems were that acceleration increased the flux of kerosene (so more small crystals of ice were available to create a slush), and there was a misconception in the heat exchanger which was aimed at avoiding water crystals.