I had this problem a couple of months ago and I couldn’t find the solution then. When i was tinkering with my development server today I noticed that all ajax widgets had stopped working. All calls stopped returning data and no errors would be shown. Checked the logs and they were all clear, left me quite dumbfounded. I had set up a new domain for the website and updated the database accordingly.
On closer examination I noticed that the calls stopped working as I used the home link. This link is a custom menu link from the new menu module (Graciously donated by the WOO crowd, many thanx for that). The link was still pointing to the old domain and thus the ajax calls stopped working. The site would still be shown since the dns entries hadn’t been removed yet. So a couple of months too late but i found the solution.