WebmasterWorld has a bug report that describes how since last Friday morning, September 15th, adCenter had a security issue. Basically, as I understand it, the conversion tracking script from Microsoft was using an expired secure site certificate. What happens when you go to a URL with an expired secure site certificate? You may get a security warning pop up on your browser. What do people do when they get a security warning? Either ignore it or close the browser.
Yesterday afternoon, adCenter411, the adCenter representative confirmed it was an issue and said it has now been resolved.
The adCenter support team has confirmed that the security certificate issue is now fixed. If any of you are still experiencing this, please let me know. I'm sorry about this inconvenience.
Almost four full days? Seems pretty big to me... It happened only to those merchants using the conversion tracking script.
Forum discussion at WebmasterWorld.
Update: Actually been an issue since the 14th... A second forum thread at WebmasterWorld with more discussion.