Ok guys i found out the problem:
When a part is later added (when there's already created content for this type) to a contentType with a datetime part and is not null the error can occur. This because it tries to read the database part record but this results in an empty datetime collumn
and raises the error.
I've added a datetime column/property to a new part and added the part to a user. Because i haven't saved the user yet (and so not create a record for the custom part) the error will be raised when i view a page because the current user is always read is
A default value doesn't do anything because the row isn't created yet. Offcourse i can make the datetime nullable but that's not what i want. it all sounds logical but it's afterwards but still it is annoying if you want to make a property not nullable.
Thanks for your support btw! Keep up the good work!