The following function appears to be where it gets stuck:

(defun gnus-id-to-thread (id)
  "Return the (sub-)thread where ID appears."
  (gnus-gethash id gnus-newsgroup-dependencies))

This function returns nil - I think that is wrong.

So, for the record (not that I really know what I am doing ;-) ),
here is the variable gnus-newsgroup-dependencies: