Hi John, 
I think I can replicate the org-ref bug now (if it is a bug).  This is the situation:

a) If I open emacs, load a file that already has some link, and click on
the link (or place the cursor on it and press enter) I get the "no key
found" message and I cannot open the notes file (I get a "Wrong type
argument: stringp, nil"), although the link to the pdf file works.  If I open more
than one file, links do not work in any of them.  At this point if I
check the value of the "org-ref-default-bibliography" variable, I get
the correct path and filename of my .bib file.

b) If I insert a new citation (using Ctrl-]) in any of the documents,
links work as they should in all documents (this is, I get the title and
I can open the notes file).  

I should say that I am not using the org-ref-insert-bibliography-link
function, because I use biblatex and I prefer to insert the Latex
\printbibliography command.  But if use it and insert the bibliography
link, the behaviour does not change.

This is what I have in my .emacs file that is related to RefTex and org-ref:

--------------------------------------------------------------------------------------
;; Load RefTex
(add-hook 'LaTeX-mode-hook 'turn-on-reftex)   ; with AUCTeX LaTeX mode
(autoload 'reftex-mode     "reftex" "RefTeX Minor Mode" t)
(autoload 'turn-on-reftex  "reftex" "RefTeX Minor Mode" nil)
(autoload 'reftex-citation "reftex-cite" "Make citation" nil)
(autoload 'reftex-index-phrase-mode "reftex-index" "Phrase mode" t)
(add-hook 'LaTeX-mode-hook 'turn-on-reftex)   ; with AUCTeX LaTeX mode
(add-hook 'latex-mode-hook 'turn-on-reftex)   ; with Emacs latex mode

;; Make RefTeX faster
(setq reftex-enable-partial-scans t)
(setq reftex-save-parse-info t)
(setq reftex-use-multiple-selection-buffers t)
(setq reftex-plug-into-AUCTeX t)

(setq reftex-default-bibliography '("/home/julian/Documents/Refs/BibTex/references.bib"))
(setq reftex-sort-bibtex-matches "author")   ; Sort entries found in BibTex database 
(setq bibtex-dialect "biblatex")

--------------------------------------------------------------------------------------
(require 'org-ref)

(setq org-ref-bibliography-notes "/home/julian/Documents/org files/notes.org"
      org-ref-default-bibliography '("/home/julian/Documents/Refs/BibTex/references.bib")
      org-ref-pdf-directory "/home/julian/Documents/Refs/")

(setq org-ref-default-citation-link "parencite")
--------------------------------------------------------------------------------------

I am sending you very simple .org and .bib files that (in my computer)
reproduce this behaviour.  In this file I did use
org-ref-insert-bibliography-link.  Let me know if I can give you any other information.  

All the best,

Julian