I have had 3 different malfunctions as a result of prolonged flights above the freezing level when carrying water ballast.

1. In an ASW 20 with a aftermarket brass valve actuated tail ballast system.. The valve froze and I was unable to dump the tail ballast. I wasn't aware of the malfunction until after landing and the valve thawed and water came running out. It was fashionable at the time to fly the ASW 20 right at or slightly behind the aft limit so dumping the wings and not the tail resulted in a dangerously tail heavy condition. I was blissfully unaware of any change in the behavior of the aircraft and landed uneventfully. The fix was to put a pint of RV antifreeze in the tail tank. RV antifreeze is propylene glycol and is safe even if you drink a little.

2. In a Nimbus 3 when flying with full wing ballast water would bubble out of the vent in the filler plugs on the wing top surface and run back over the flap aileron junction on the trailing edge and freeze, resulting in frozen controls. It was quite alarming but not too hard to dislodge with a vigorous thrashing of the controls. I learned to cycle that gap frequently to break it off before it became thick enough to be a problem.

3. I have always used the wax from wax toilet mounting rings to seal the dump valves on Schempp Hirth gliders. After a long wave flight in NZ in my Nimbus 3 I landed at Omarama and promptly did a wild ground loop. Omarama is a wide grass field and the glider escaped unscathed. On most airport runways I probably would have done heavy damage. The wax had become so stiff that one valve stayed stuck shut, and there was enough slack in the dump linkage to allow the cockpit dump handle to stow in the dump position, so I had no way of knowing that one wing was not dumping. Interestingly, although I had been flying for some time with one wing empty and the other full, I was not aware of the asymmetry until the heavy wing went down on the landing. The solution was to switch to chap stick which stays fairly soft when cold. It doesn't seal as well but is a wiser choice.
I guess one could conclude that when flying for extended times, in below freezing temperatures, with water ballast it is advantageous to have been born lucky.

DLB