Today, I tried to create a new request leave but suddenly the app displayed this error

Can anyone give me an idea about possible sources of this error ?

Traceback (most recent call last):
  File "/opt/odoo/odoo/openerp/http.py", line 544, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/opt/odoo/odoo/openerp/http.py", line 581, in dispatch
    result = self._call_function(**self.params)
  File "/opt/odoo/odoo/openerp/http.py", line 317, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/opt/odoo/odoo/openerp/service/model.py", line 118, in wrapper
    return f(dbname, *args, **kwargs)
  File "/opt/odoo/odoo/openerp/http.py", line 314, in checked_call
    return self.endpoint(*a, **kw)
  File "/opt/odoo/odoo/openerp/http.py", line 810, in __call__
    return self.method(*args, **kw)
  File "/opt/odoo/odoo/openerp/http.py", line 410, in response_wrap
    response = f(*args, **kw)
  File "/opt/odoo/odoo/addons/web/controllers/main.py", line 1247, in load
    action = request.session.model(action_type).read([action_id], False, ctx)
  File "/opt/odoo/odoo/openerp/http.py", line 911, in proxy
    mod = request.registry[self.model]
  File "/opt/odoo/odoo/openerp/modules/registry.py", line 102, in __getitem__
    return self.models[model_name]
KeyError: u'Email'