The whole problem was caused by a validations in my details model - I have validations that are checking both the format and the presence of some of the fields, so apparently rails did not allowed me to created object with empty fields.
To fix this I have to used validate:false.
I have lost a lot of time reading about rails defaults like
config.active_record.whitelist_attributes = true
and the use of attr_accessible, but it seems that failed validations can threw the same
Can't mass-assign protected attributes
error.