EPiServer - ReflectionTypeLoadException: Unable to load one or more of the requested types

Today I installed Page Type Builder 1.2 Beta 2 on my ongoing EPiServer CMS 6 project. I dropped the Page Type Builder assemblies in the bin folder, recreated the references, did an iisreset and then tried to start the site. I got the following exception:

Unable to load one or more of the requested types. The following information may be a subset of the Type/LoaderException information present - inspect with debugger for complete view.
Check assemblies [PageTypeBuilder, Version=1.1.9.1, Culture=neutral, PublicKeyToken=6fb8762af0e6dbed] and/or types [PageTypeBuilder.Reflection.MethodInfoExtensions,
PageTypeBuilder.Abstractions.PageTypeFactory,
PageTypeBuilder.Activation.PageTypePropertyInterceptor,
PageTypeBuilder.Discovery.PageTypeDefinition,
PageTypeBuilder.Synchronization.PageTypePropertyUpdater,
PageTypeBuilder.Discovery.PageTypePropertyDefinitionLocator,
PageTypeBuilder.Initializer,
PageTypeBuilder.Synchronization.PageTypeUpdater,
PageTypeBuilder.PageTypeBuilderException,
PageTypeBuilder.UI.PageDataValidationExtensions,
PageTypeBuilder.Synchronization.Validation.UnmappablePropertyTypeException,
PageTypeBuilder.Discovery.PageTypeDefinitionLocator,
PageTypeBuilder.PageDataExtensionMethods,
PageTypeBuilder.Activation.PageTypePropertiesProxyGenerationHook,
PageTypeBuilder.Synchronization.TabDefinitionUpdater,
PageTypeBuilder.Reflection.AttributedTypesUtility,
PageTypeBuilder.Discovery.PageTypePropertyDefinition,
PageTypeBuilder.Synchronization.TabDefinitionExtensions,
PageTypeBuilder.Synchronization.PageTypeSynchronizer,
PageTypeBuilder.Synchronization.PageTypePropertyDefinitionExtensions]. Information from LoaderExceptions property [Request for the permission of type 'System.Web.AspNetHostingPermission, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.,
Request for the permission of type 'System.Web.AspNetHostingPermission, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.,
Request for the permission of type 'System.Web.AspNetHostingPermission, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.,
Request for the permission of type 'System.Web.AspNetHostingPermission, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.,
Request for the permission of type 'System.Web.AspNetHostingPermission, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.].

The reason for this exception is Windows 7 security features. When you download assemblies from Internet they might be marked as unsafe. To solve this, right click each assembly, select Properties and click the Unblock button on the General tab.