I use org-mode all the time and find it invaluable. I recently upgraded my
ubuntu environment to 13.04 and emacs 24.2.1 came with it. org-mode was
working fine within it out of the box except it failed with "Symbol's
function definition is void: org-defvaralias" when I restarted the machine
this morning.

Not finding much on google I decided to upgrade org-mode to the latest
release so I added
(require 'package)
(add-to-list 'package-archives '("org" . "http://orgmode.org/elpa/") t)
(unless package-archive-contents    ;; Refresh the packages descriptions
  (package-refresh-contents))
(setq package-load-list '(all))     ;; List of packages to load
(unless (package-installed-p 'org)  ;; Make sure the Org package is
  (package-install 'org))           ;; installed, install it if not
(package-initialize)                ;; Initialize & Install Package

to my init as per http://orgmode.org/elpa.html#. This gave me org-version
8.0.3

Now my agenda is completely blank with no tasks displaying at all (they
also fail to display in the other views).

A message "org-agenda-skip: Wrong type argument: stringp, nil" appears when
I try to recalculate the agenda view.

I know that something is now failing in one of my org files because if I
reset org-agenda-files to a simple test file it works. However, I have
literally dozens of org files, some of which are many thousands of lines
long because I use them for client documentation. They were working fine in
the previous version of org-mode.

Another, possibly related issue is that when emacs opens and the agenda is
calculated, the test file buffer is marked as changed when it is not
changed.

Is there some way I can easily narrow down the issue or perhaps restore the
robustness of the previous org-mode version?