Tom, I don't believe hydraulic fluid will "freeze" easily and certainly not at the temperatures you had. Sadly though, colder temperatures will cause the tubing and fittings to shrink a bit and this contraction can be unequal between dissimilar materials, opening a path for the fluid to move through.
It appears to me that the fitting you show is the return line to the reservoir, which is not under any significant pressure. It should only have fluid in it when you lower the top. Is that when the leakage occurred? If it was losing fluid with the top up, that signals a likely leak at the hydraulic ram. The seals there can also shrink in the cold and through disuse. If this is the case, you will probably eventually reach a leakage rate where the top will no longer raise fully and the seals will have to be repaired. Any hydraulic shop can do this, I think. Others in the forum have had to make this repair.
As far as the leak you observed, possibly you could tighten that fitting just a wee bit? Or, the transparent tubing might have to be replaced, and either job is DIY, I believe.
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