Hi,
this is a Joomla routing problem, if you have menu item of type "Users Manager->registration form" and not have menu item like "Users Manager->User Profile" or "Users Manager->edit Profile" then Joomla redirect Edit profile on Registration menu Item.

How to solve?
set Access level for "Users Manager->registration form" menu item to "Guest"

Thats not the case.

The user is already registered and a member for several days. He completed his registration and is approved.

Now we add a new required field... The user logs in, gets redirected to the profile to fill in this required field. When he does, the system tells him that the email is already in use.

Yes this is the case, when user make login, he will be redirect at this url: http://www.siteatelier.nl/registreer/profile?layout=edit

I see registreer in the URL, try to set access level for this menu item to "Guest"

you are allowed to slap me in the face for this!

Thnx...