ambiguous reference

It sounds like your project has two different references to the same assembly.

Get rid of one of them.

Have you tried right-clicking the solution and doing Clean Solution?

Have you tried turning it (Visual Studio) off and on again? It worked for me...

I was getting this as an erroneous error message, and my project still built and ran fine for months. It was because someone had put a class in a namespace with different capitalisation to all the other instances of that namespace, so they were effectively two different namespaces. Our code was technically correctl, but the ASPX <%@ Register %> directive complained that it was ambiguous because there were two different symbols with the "same" name; MyNamespace and Mynamespace.

Check your capitalisation.