Default page server error 'unrecognized attribute'

When a user browses to the default page of ArchiOffice/EngineerOffice, it displays a server error ‘unrecognized attribute…’ in the application.

The error occurs during the processing of a configuration file required to service this request. 

You can correct this problem by taking the following steps:

1. Open the IIS Manager from Start > Control Panel > Administrative Tools > Internet Information Services.
2. On the left, click to expand your local computer name-Sites. Right-click on AO20xx and select Properties.
3. Move to the ASP.NET tab. For ASP.NET version, select 4.x (say 4.0.30319).
4. Click OK.