Cfrtjryk

After having read this answer I wondered what happens if hydrazine freezes.

It looks like it is "normal" and not like water, in the sense that it contracts when freezing, unlike water which expands and can cause mechanical damage.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a042039.pdf

Sometimes hydrazine thrusters are used after several years, and an extreme example would be the reboot of ISEE-3/ICE. But that was circa 1 AU. Sometimes hydrazine is used in spacecraft much farther away; Voyager 1 Fires Up Thrusters After 37 Years, and I am not sure if it could have keep its hydrazine above the melting point all this time, but I don't know.

Per answers there, it seems that Voyager's hydrazine might be kept above freezing, but that doesn't necessarily mean that freezing of hydrazine in general would be catastrophic, but only that it's easier to keep it liquid than to set up some special system to thaw it when needed.

How cold are the Voyagers now? Colder than LOX? Colder than SOX?

At least for the Space Shuttle, freezing was OK, but thawing out was bad for piping. Hydrazine contracts when it freezes, so it can 'superpack' (more fluid flows in, then freezes, etc.)...then when it thaws out, there is more than can fit in the pipe, and it can burst.

In the Space Shuttle's auxiliary power unit, hydrazine plumbing was allowed to sustain two freeze/thaw cycles, but then it was considered lost.

Reference: Space Shuttle flight rules, page 10-5

The Olympus satellite (1989-053A, 20122) lost pointing and power for long enough that all the fuel froze. It was recovered after a couple months and the fuel defrosted. I couldn't easily find what fuel was used, but it must either be hydrazine or a derivative. The 21-Sep-1991 New Scientist article Nine-week battle that saved Olympus explains:

Engineers from the European Space Agency recounted this week the dramatic
64-day rescue of the agency’s Olympus satellite which, at the end of May,
was spinning out of control, its fuel frozen solid and its batteries flat.

[...]The satellite was spinning once every 90 seconds, its temperature plunged
to below -50 °C and it was drifting eastwards at 5 degrees per day.
Although, when control was lost, its solar array was pointing away from
the Sun, the team knew that light would fall on the array as the Earth rotated
around the Sun. By 19 June, the array had turned enough for there to be
sufficient power for it to respond to a command beamed up from Perth in
Australia.

On 1 July, there was enough power in the batteries to turn the array
fully towards the Sun and, with that done, all the batteries were charged
by 8 July. With power restored, it was possible to thaw out the fuel and
thrusters that control Olympus’s position. The engineers feared that pipes
and valves blocked with frozen fuel might have burst, irreparably damaging
the satellite. But test firings on 26 July went well.

This is a minor elaboration on Organic Marble's and Ross Milikan's answers. The driving principle is the same superpack (I've not heard it called that before) concern as in Organic Marble's answer.

There is a difference in that the freezing points of Hydrazine, Nitrogen Tetroxide and Mono Methyl Hydrazine are quite different (very roughly 2 deg C, ~-15 degC and -45deg C respectively), but its the same process: cold->freezing->contraction->more fluid (under pressure) fills the void-> thawing and expansion of a locked void.

My contribution: the remedy is to attempt to thaw only the pipe zones that are adjacent to warmer, liquid, sections of pipe first. This is only going to work if the heaters for the pipework allow it.