by house master.
Ok it's working now. I enabled the developer debug messages and it told me I forgot to pass the parameters of the custom profile files, which were mandatory as well.
If anyone wonders how to pass multiple custom fields:
&customprofilefields[0][type]=string&customprofilefields[0][name]=profile_field_NAME&customprofilefields[0][value]=placeholder&customprofilefields[1][type]=string&customprofilefields[1][name]=profile_field_NAME2&customprofilefields[1][value]=placeholder
If anyone wonders how to pass multiple custom fields:
&customprofilefields[0][type]=string&customprofilefields[0][name]=profile_field_NAME&customprofilefields[0][value]=placeholder&customprofilefields[1][type]=string&customprofilefields[1][name]=profile_field_NAME2&customprofilefields[1][value]=placeholder
...but now I have to confirm the user and don't know the "secret" parameter. Anyone knows how to get that one?
The function I'm using is core_auth_confirm_user, which requires the username and the secret.