ASP.Net Website project fails to build after updating a referenced DLL

So I have an ASP.NET VB website that references several other projects (their DLLS are just put in the site’s bin folder). I need to update a small piece of code in one of the projects, which I have done and it builds fine. However, when I copy over the new DLL to the website’s bin folder it fails to build, and all the Imports statements say “BC40056: Namespace or type specified in the Imports xxx doesn’t contain any public member or cannot be found”, which results in a ton of errors like “is not defined” . There is no reference to the updated DLL in the website project’s properties, but if I put the old version back it’s all fine.

Reference DLLs in ASP.NET without Bin or GAC

I have an ASP.NET project under source control (Subversion). For various reasons, I don’t want to add the Bin directory or its contents to source control, so I have it svn:ignored. DLLs are loaded into here during a Visual Studio build, and I can start with a clean directory and/or delete all the contents of this directory and still have a successful build.