This is just a guess from the traceback (which seems to be interleaved between multiple request).
It looks like the sessions, returned by the search in Session.check, contain an instance with a negative (or None) id. So when it tries to access create_date still in Session.check, Model.__getattr__ raise AttributeError which is re-raised in ModelStorage.__getattr__.
Normally, there is a not null constraint on id so I guess it is the negative one.
It will be good to add a constraint against non negative id on the table (maybe even in base).