Thanks for your reply. Receiving the following error in Safari: "Resource interpreted as script but transferred with MIME type application/json." The above message does not appear when the Twitter module is disabled. Issue is also occurring in IE8 and Chrome, yet no javascript errors as far as I can tell. Firefox is working OK.
Was having same issue - all products with combinations were displaying message ""The product does not exist in this model. Please choose another." Disabled Twitter module, issue resolved. Has anyone found a way to get combinations, and the twitter module working together?