This damn thing seems to crop up whenever you least expect it. There's multiple solutions, none guaranteed.
Essentially, the initial problem with be a message in big red letter which says "Server Application Unavailable"
This is basically a message from IIS saying you don't have permissions to see the proper error.
In my particular case, giving the directory hosting the web app full security permissions to the ASPNET user identity and dropping the IIS Virtual Directory (tab) Application Protection to Low allowed the true error to be seen.
This was: Could not load file or assembly 'System.Web.Extensions, Version=2.0.....' etc.
At the same time, the Event Viewer also started showing: Failed to execute the request because the ASP.NET process identity does not have read permissions to the global assembly cache
I have also tried referencing the correct DLLs in the project references as I had tried to correct these links to the up-to-date DLLs, however they should have been pointing at a very specific location, brought in by SVN.
Ok, so the secret to this particular mess seems to have been "Make sure you're referencing the right DLLs."
Now the only problem is to solve the code issues in the controls!
Resources looked at so far include: