After upgrading my DNN5 site to DNN 07.04.01 the user registration process throws an error: "An error has occurred. Error: is currently unavailable."

The registration mode is 'verified'. When I change the mode to 'public', everything works fine. 
You can experience it yourself: http://www.e-learning.nl/register.aspx

This is a major issue for us. So hoping that someone can help :)

ModuleId:-1

ModuleDefId:-1

FriendlyName:

ModuleControlSource:DesktopModules/Admin/Security/Register.ascx

AbsoluteURL:/Default.aspx

DefaultDataProvider:DotNetNuke.Data.SqlDataProvider, DotNetNuke

ExceptionGUID:e9157007-3aac-4c05-b91a-b3d57f2622f0

AssemblyVersion:7.4.1

PortalId:0

UserId:-1

TabId:41

RawUrl:/register.aspx

Referrer:http://www.e-learning.nl/register.aspx

UserAgent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_1) AppleWebKit/601.2.7 (KHTML, like Gecko) Version/9.0.1 Safari/601.2.7

ExceptionHash:ZnESpqiGwPjkmJASt9/zXQ==

Message:Object reference not set to an instance of an object.

StackTrace:

InnerMessage:Object reference not set to an instance of an object.

InnerStackTrace:

   at DotNetNuke.Entities.Modules.UserModuleBase.SendAdminNotification(UserInfo newUser, PortalSettings portalSettings)
   at DotNetNuke.Entities.Modules.UserModuleBase.CompleteUserCreation(UserCreateStatus createStatus, UserInfo newUser, Boolean notify, Boolean register)
   at DotNetNuke.Modules.Admin.Users.Register.CreateUser() in d:\sites\vhosts\e-learning.nl\httpdocs\DesktopModules\Admin\Security\Register.ascx.cs:line 320

I use the Host SMTP settings. 

I don't understand why this fails after upgrading to DNN7. Before (DNN5) verified user registration works fine!
And I noticed localizations aren't fully implemented. Several text labels are in English now. In DNN5 they we in Dutch :(

When I search for this issue, I found several threads, but without solution. It looks like a 'known' problem with the upgrade from DNN5 to DNN7.