Hey there, the roots got contaminated with water mold from not enough drainage, take a pair of sizors and just cut off most of the problem roots (max 20% of the total root mass), then use something like enzymes, humic acid or a diluted flushing agent to try and fix the rest. Basically it's a humidity and temperature issue if you're at 27C + high humidity and just a bit of overwatering can cause issues. Next time use a chelation agent every watering or add beneficial bacteria like bacilus subtilis in the soil from the start and sometimes this will help prevent the problem you're seeing. Hope this helps ! 🚀