the following error message I get every time an email is to be sent

Traceback (most recent call last):
  File "/opt/odoo/odoo-server/openerp/http.py", line 530, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/opt/odoo/odoo-server/openerp/http.py", line 567, in dispatch
    result = self._call_function(**self.params)
  File "/opt/odoo/odoo-server/openerp/http.py", line 303, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/service/model.py", line 113, in wrapper
    return f(dbname, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/http.py", line 300, in checked_call
    return self.endpoint(*a, **kw)
  File "/opt/odoo/odoo-server/openerp/http.py", line 796, in __call__
    return self.method(*args, **kw)
  File "/opt/odoo/odoo-server/openerp/http.py", line 396, in response_wrap
    response = f(*args, **kw)
  File "/opt/odoo/odoo-server/addons/web/controllers/main.py", line 935, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "/opt/odoo/odoo-server/addons/web/controllers/main.py", line 927, in _call_kw
    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/api.py", line 336, in old_api
    result = method(recs, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/addons/base/res/res_partner.py", line 573, in create
    partner = super(res_partner, self).create(vals)
  File "/opt/odoo/odoo-server/openerp/api.py", line 239, in wrapper
    return new_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/api.py", line 462, in new_api
    result = method(self._model, cr, uid, *args, **kwargs)
  File "/opt/odoo/odoo-server/addons/mail/mail_thread.py", line 384, in create
    self.message_post(cr, uid, thread_id, body=_('%s created') % name, context=context)
  File "/opt/odoo/odoo-server/openerp/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/addons/mail/mail_thread.py", line 1673, in message_post
    msg_id = mail_message.create(cr, uid, values, context=context)
  File "/opt/odoo/odoo-server/openerp/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/addons/mail/mail_message.py", line 808, in create
    user_signature=context.get('mail_notify_user_signature', True))
  File "/opt/odoo/odoo-server/openerp/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/addons/mail/mail_message.py", line 877, in _notify
    force_send=force_send, user_signature=user_signature
  File "/opt/odoo/odoo-server/openerp/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
TypeError: _notify() got an unexpected keyword argument 'user_signature'

Unfortunately I can not find the causeUnfortunately I can not find the cause 

EDIT:

@zbik
I forgot to mention, sorry. That was my first thought, to set the signature. But the signature is set. (my karma is to low to comment)