I'm sorry it took so long for me to get here, I will do my best to answer this question. Newts are sensitive to ammonia levels in the water, and with their feeding habits the way they are, they need more frequent water changes than a standard aquarium. Newts also need dry land to climb onto when they feel the need. Newts are dirty little things when it comes to waste levels. The white sores you describe sound like something I've seen all too frequently, which is unfortunate, because it is easily preventable with the proper set up and care. The ammonia levels rise, which means nitrite and nitrate levels rise, and often we don't realize it. When this goes on for a period of time, dependent on the number of animals vs size of tank, amount of water, ect etc., the pH in the water will begin to drop. The effects of the water chemistry have some drastic effects on the body of the newts, and they are apparent as the white sores you witnessed. Those sores quickly become infected, as newts don't have much for an immune system to begin with.
There are a few treatments if it is caught in the early stages and corrected immediately with a fresh new setup, meaning breaking down the tank and doing a thorough rinsing of everything in the setup. A liquid antiseptic usually found over the counter at most local drug stores, called Betadine, can be dabbed onto the sores using a cotton swab. This is done twice/day until the sores heal. The medication and clean conditions can save most newts if the problem is dealt with immediately.
I hope this was helpful, feel free to post further questions.