This error cropped up while writing some Methods to modify existing Active Directory user accounts. This error occurs if you try to use DirectoryEntry.CommitChanges to update the CN attribute of an entry object. If you search around a little you'll find the short answer is to use the DirectoryEntry.Rename method for changing the name. However, it's important to note that the Rename method needs the format "CN=Joe Smith". For example.
objEntry.Rename("CN=" & strFirstName & " " & strLastName)

I've been getting this error intermittently for a while now and finally got tired of seeing it, so I started looking for causes and solutions. The exact error I was getting is shown below. Note that it could appear with the same error 'BC30560', but have a different class name and different namespace.

Compiler Error Message: BC30560: 'Login_ascx' is ambiguous in the namespace 'ASP'

I couldn't really find a clear answer as it seems it can be caused by different scenarios, some of which may be a result of a bug in the framework. In my case it was a simple fix, although I don't know why this fixed it.

I had a page named Login.aspx and a User Control that resided on that page named Login.ascx. Why did I do it this way? Because I used the Login.aspx page as the primary login page, but there were other scenarios where I wanted to show a login without redirecting to the login page. So I created a User Control that I could put anywhere I wanted.

It seems as though there is a bug in the asp.net framework since my page and my control have different file extensions and different class names. Yet as soon as I changed my User Control to be named LoginCtrl.ascx, the error stopped appearing.