ActiveAndroid: All Date columns null after restarting app

I am trying to store a Person object with ActiveAndroid. I set the birthdate like this: person.setBirthdate(new java.sql.Date((new Date()).getTime()));. After assigning all data i am calling person.save();.

I get a list of all persons from the database with this command: new Select().from(Person.class).execute();

If I do not close the app everything works fine (birthdate is always returned correctly from the database). However, when I close the app completely and then restart it, the birthdate is always null. All other fields (first name, last name etc.) are correct.

I really don't know why this is happening and would appreciate some help!