The Active Directory datatype cannot be converted to/from a native DS datatype

The nice sentence above gave me quite a headache today… as I was deploying the authentication for web app, it blew up with this rather cryptic message, as did every other authenticated web app installed on the machine.

The solution is very simple, so if you happen to get hit by this one, the problem is essentially that the type cache has gone corrupted in a rather typical windows manner, so you have to force it to be rebuilt. You do this by going to %WINDOWS%/SchCache and deleting the files in that directory. Reboot afterwards, and you’re done :)