Hi,
I'm still experiencing this bug, although with a slightly different error message. When "C-c C-c" on a headline, I see:
org-set-tags: Wrong type argument: listp, org-tags-completion-function
Compared to the original report of:
apply: Wrong type argument: listp, org-tags-completion-function
I'm running latest git head:
Org-mode version 8.3beta (release_8.3beta-1078-gd13a2b @ /Users/mankoff/local/src/org-mode/lisp/)
-k.