I have not come across this particular product before.
However, most Chemicals are more soluble at higher temperatures, (you can dissolve sugar in hot Tea or Coffee much more easily than you can in cold water, the same with table salt). So, sometimes, if the Chemical is stored at too low a temperature, or for too long a time, the Chemicals can come out of solution and form crystals.
Generally the solution would be to gently warm the liquid and stirr it to re dissolve the crystals but beware of the Flash point when you are heating the chemical!! That is the simplest solution. Find a warm spot in the engine room where there is lots of vibration and store the drums there for a while.
Alternatively, the fuel treatment should be soluble in Gas Oil, so pouring off the liquid phase of the treatment and then dissolving the remaining crystals in Gas Oil should work, although then it becomes harder to calculate your dosage rates. You would have to make sure tou used an equal % of the original liquid and the crystsals dissolved in Gas Oil, the Chemical composition of the remaining liquid may not be the same as that of the crystals. There may be several different Chemicals dissolved the original solution, not all of which became "super saturated" and crystalised out.
It may also be possible that some of the solvent has evaporated, if the liquid level in the drum is low, in which case adding Gas Oil should do the trick.
Hope the information helps, Good luck.
It is always better to ask a stupid question than to do a stupid thing.