If the AD webservice is installed on a SharePoint site, some SharePoint updates or admin operations may have affected the app pool assignment. Make sure the app pool assigned is DbxlAppPool{SITEID} {SITEID} will be the ID of the site. It should show as Classic managed pipeline mode if on IIS7.
If you remote desktop to the server, enter the url to the AdUserInfo.asmx page (e.g. http://server/QdabraAD/AdUserInfo.asmx) and then click on the GetMyInfo link and then click Invoke. You should get more error detail.
I would also check the Qdabra eventlog for additional details.