Value cannot be null. Parameter name: type. ProfileBase.CreateMyInstance.

Hi,
We are currently stuck on the following error, whenever any of our code makes a call to the ProfileCommon / ProfileBase class(es).
The full stacktrace looks like this:
 [ArgumentNullException: Value cannot be null.
Parameter name: type]
   System.Activator.CreateInstance(Type type, Boolean nonPublic) +7467190
   System.Web.Profile.ProfileBase.CreateMyInstance(String username, Boolean isAuthenticated) +79
   System.Web.Profile.ProfileBase.Create(String username, Boolean isAuthenticated) +231
   System.Web.HttpContext.get_Profile() +107
   OurApp.Login.get_Profile() +17
 
Some background:
Our application has been running fine for the last few years. This past week we have added a new property to our ProfileProvider (new entry in the web.config, a DateTime value). We've made some modifications to support this new property in the code (lo ...