Find the directories owned by root in a certain part of the tree:

# find . -depth -mindepth 1 -maxdepth 3 -type d -ls | awk '$5 ~ /root/ {print}'
  7930    0 drwxr-xr-x  12 root root      115 Oct 11 16:44 ./562
3805069    0 drwxr-xr-x   3 root root       20 Oct 11 16:44 ./562/8562
  7946    0 drwxr-xr-x   5 root root       46 Dec  8 23:52 ./563/6563
  7947    0 drwxr-xr-x   3 root root      21 Oct 21  2008 ./563/6563/456563
3464735    0 drwxr-xr-x   2 root root        6 Sep 26 17:29 ./563/6563/436563
4075144    4 drwxr-xr-x   2 root root     4096 Dec  9 00:39 ./563/6563/2366563

Change all the ownership to www-data:

# find . -depth -mindepth 1 -maxdepth 3 -type d -exec chown www-data: {} \;

You could do this:

# cd .. && chown -R www-data: dirname

But we only suspect the problem at a certain level in the tree, and it would be way slow to recursively chown hundreds of millions of files.