Why all the emphasis on
unknown problem (where did the water come from)
?
With respect, we knows where the water in fuel tanks (ground based or airborne) comes from - it comes from the atmosphere - either warm wet air being cooled, or suspended moisture AKA cloud.
Descending through cloud with a large empty centre tank for instance will introduce water in some form. If the tank is cold, the moisture will condense as frost all over the inner surface.
The question is not where does the water (or ice) come from, but how and where does so much manage to successfully accumulate, and yet be suddenly released in such volume that it can bring an aircraft down?
The sudden release looks like melting. The ability to bring the aircraft down looks like refreezing. The path from warm(?) centre tank to engine passes through the (cold?) main tanks.
Other possibilities may exist - that is simply one obvious sequence along the fuel path.
The question is which possibility actually became a reality?