Seems that there will be an intermediate version (2.65) of nopCommerce released in the near time because of an issue in FluentValidation described here and here.
In brief, the previous version FluentValidation library (and hence nopCommerce) does not work on servers with ASP.NET 4.5 installed. It should be done as soon as possible because many servers (on shared hosting) will be upgraded to ASP.NET 4.5 in the near time