Hello,
Could you please check your web.config file, if you are using the correct one?
Specifically, if there is the prefix "cms" registered in the following form:
<add tagPrefix="cms" namespace="CMS.Controls" assembly="CMS.Controls" />
Could you please try to open the project as a web site, not through the *.sln file?
Please check, if you are using the correct dll files for the .net version 4.0.
Best regards,
Boris Pocatko