I'm no expert on this stuff, but this happened to me one time when I was firing up the 16-gallon UDS. This thing normally warms up quick and when it does, goes right to 'thin blue' smoke. I learned something this day...
I had filled the charcoal basket (K-briquettes), then lit my usual 6-10 briquettes and placed them on top of the basket. At this point I thought, "I can get more charcoal in this thing than that!" and proceeded to pile on more briquettes. At most there was only 1 or 2 more layers of briquettes on top of the already burning ones.
Everything seemed fine, but after an hour it was still chugging PLUMES of thick white smoke! I ended up letting it go for 90 minutes until the smoke started to thin down, and when I put the PORK in the drum, I noticed that was because all of that top-charcoal had become lit. Once it lit things were fine.
Maybe some unburnt coals are collapsing into the center (lit) section causing the smoke? To me, thick white smoke means that the fire isn't hot enough, or isn't getting enough 'draft' (airflow).
Just some thoughts to possibly help...