Some rabbits are prone to urine sludge and its not fully understood why. It might be a genetic predisposition rather than a sub-par diet. All rabbits should expel surplus calcium via the urine, whereas most mammals (incl humans and dogs) get rid of surplus calcium via the feces.
Cloudy urine is thus not considered a problem. Its actually worse if the urine is clear, because it means the calcium carbonate crystals collect in the bladder.
Rabbits can get urinary tract infections just like humans can. A UTI can cause a fever and dark, cloudy urine. The rabbit often strains to pee and/or pees often (dribble). Urination outside the litterbox is typical.
For a UTI, antibiotics are recommended. Hydration is very important. Serve veg freshly rinsed and dripping wet. Cooled hay “tea” or a small amount of fruit juice added to the bun’s water might encourage the bun to drink more. Always serve plain water as well, some rabbits refuse to drink water that tastes “funny”.