Hi Giles,
This might help - Just going to the checkout page regardless (shows the store account module), leads to this error also. Bit more explanation this time hope it means something useful :)
An unexpected error has occurred!
DotNetNuke.Services.Exceptions.ModuleLoadException: The type initializer for 'DotNetNuke.Modules.Store.Components.SymmetricHelper' threw an exception. ---> System.TypeInitializationException: The type initializer for 'DotNetNuke.Modules.Store.Components.SymmetricHelper' threw an exception. ---> System.Configuration.ConfigurationErrorsException: An error occurred loading a configuration file: Request for the permission of type 'System.Security.Permissions.FileIOPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed. (machine.config) ---> System.Security.SecurityException: Request for the permission of type 'System.Security.Permissions.FileIOPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed. at System.Security.CodeAccessSecurityEngine.Check(Object demand, StackCrawlMark& stackMark, Boolean isPermSet) at System.Security.CodeAccessPermission.Demand() at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy) at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share) at System.Configuration.Internal.InternalConfigHost.StaticOpenStreamForRead(String streamName) at System.Configuration.Internal.InternalConfigHost.System.Configuration.Internal.IInternalConfigHost.OpenStreamForRead(String streamName, Boolean assertPermissions) at System.Configuration.Internal.InternalConfigHost.System.Configuration.Internal.IInternalConfigHost.OpenStreamForRead(String streamName) at System.Configuration.Internal.DelegatingConfigHost.OpenStreamForRead(String streamName) at System.Configuration.UpdateConfigHost.OpenStreamForRead(String streamName) at System.Configuration.BaseConfigurationRecord.InitConfigFromFile() The action that failed was: Demand The type of the first permission that failed was: System.Security.Permissions.FileIOPermission The Zone of the assembly that failed was: MyComputer --- End of inner exception stack trace --- at System.Configuration.ConfigurationSchemaErrors.ThrowIfErrors(Boolean ignoreLocal) at System.Configuration.BaseConfigurationRecord.ThrowIfParseErrors(ConfigurationSchemaErrors schemaErrors) at System.Configuration.Configuration..ctor(String locationSubPath, Type typeConfigHost, Object[] hostInitConfigurationParams) at System.Configuration.Internal.InternalConfigConfigurationFactory.System.Configuration.Internal.IInternalConfigConfigurationFactory.Create(Type typeConfigHost, Object[] hostInitConfigurationParams) at System.Web.Configuration.WebConfigurationHost.OpenConfiguration(WebLevel webLevel, ConfigurationFileMap fileMap, VirtualPath path, String site, String locationSubPath, String server, String userName, String password, IntPtr tokenHandle) at System.Web.Configuration.WebConfigurationManager.OpenWebConfigurationImpl(WebLevel webLevel, ConfigurationFileMap fileMap, String path, String site, String locationSubPath, String server, String userName, String password, IntPtr userToken) at System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration(String path) at DotNetNuke.Modules.Store.Components.SymmetricHelper.Initialize() at DotNetNuke.Modules.Store.Components.SymmetricHelper..cctor() --- End of inner exception stack trace --- at DotNetNuke.Modules.Store.Components.SymmetricHelper.Encrypt(String unicodeValue) at DotNetNuke.Modules.Store.Cart.CurrentCart.setCartID(Int32 portalID, String cartID) at DotNetNuke.Modules.Store.Cart.CurrentCart.getCartID(Int32 portalID) at DotNetNuke.Modules.Store.Cart.CurrentCart.GetItems(Int32 portalID) at DotNetNuke.Modules.Store.WebControls.CustomerCart.CheckStateButton() at DotNetNuke.Modules.Store.WebControls.CustomerCart.Page_Load(Object sender, EventArgs e) at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) at System.Web.UI.Control.OnLoad(EventArgs e) at DotNetNuke.Modules.Store.Components.StoreControlBase.OnLoad(EventArgs e) --- End of inner exception stack trace ---