I got this error when installing KES onto my server.
The solution is the following (or was for me)

Run - aspnet_regiis -i

from the .Net Framework directory:

If you don't have a directory named like that,
you don't have the .Net Framework 2.0 installed.

I had the directory, however framework 2 was not registered with IIS which is what caused the problem.

Legacy Forum Name: ASP Net - Unrecognized attribute 'xmlns' - Solution,
Legacy Posted By Username: mrbiggles