Hi Bhags,
In Ayurveda explains curd (Dadhi) as having sour mixed sweet property and it increase Kapha dosha(characteristics of Kapha include heaviness, softness, sweetness, cold, stability) in the body. the mucus generation is also attributed to the effect of Kapha. and during night period, ther is natural pridominance of Kapha in the body. So, curd consumption at night will further increase kapha leading to many complications.
This is quite similar to exposing yourself to hot Sun in the afternoon hours, which is not reccomended, because, afternoon period is naturally Pitta pridminant period and sun exposure also increases Pitta.