When I try to Unreconcile customer payment I get a following error:

>>>
2014-07-10 19:27:55,381 11652 ERROR mycompany openerp.http: Exception during JSON request handling.
Traceback (most recent call last):
  File "/Users/janno_liivak/development/python/odoo/openerp/http.py", line 476, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/Users/janno_liivak/development/python/odoo/openerp/http.py", line 495, in dispatch
    result = self._call_function(**self.params)
  File "/Users/janno_liivak/development/python/odoo/openerp/http.py", line 311, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/Users/janno_liivak/development/python/odoo/openerp/service/model.py", line 113, in wrapper
    return f(dbname, *args, **kwargs)
  File "/Users/janno_liivak/development/python/odoo/openerp/http.py", line 308, in checked_call
    return self.endpoint(*a, **kw)
  File "/Users/janno_liivak/development/python/odoo/openerp/http.py", line 685, in __call__
    return self.method(*args, **kw)
  File "/Users/janno_liivak/development/python/odoo/openerp/http.py", line 360, in response_wrap
    response = f(*args, **kw)
  File "/Users/janno_liivak/development/python/odoo/addons/web/controllers/main.py", line 987, in call_button
    action = self._call_kw(model, method, args, {})
  File "/Users/janno_liivak/development/python/odoo/addons/web/controllers/main.py", line 975, in _call_kw
    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
  File "/Users/janno_liivak/development/python/odoo/openerp/api.py", line 204, in wrapper
    return old_api(self, *args, **kwargs)
  File "/Users/janno_liivak/development/python/odoo/addons/account_voucher/account_voucher.py", line 952, in cancel_voucher
    reconcile_pool.unlink(cr, uid, [line.reconcile_id.id])
  File "/Users/janno_liivak/development/python/odoo/openerp/api.py", line 204, in wrapper
    return old_api(self, *args, **kwargs)
  File "/Users/janno_liivak/development/python/odoo/addons/account/account.py", line 1617, in unlink
    return super(account_move_reconcile, self).unlink(cr, uid, ids, context=context)
  File "/Users/janno_liivak/development/python/odoo/openerp/api.py", line 204, in wrapper
    return old_api(self, *args, **kwargs)
  File "/Users/janno_liivak/development/python/odoo/openerp/models.py", line 3533, in unlink
    recs.recompute()
  File "/Users/janno_liivak/development/python/odoo/openerp/api.py", line 202, in wrapper
    return new_api(self, *args, **kwargs)
  File "/Users/janno_liivak/development/python/odoo/openerp/models.py", line 5449, in recompute
    f.name: rec[f.name] for f in field.computed_fields
  File "/Users/janno_liivak/development/python/odoo/openerp/models.py", line 5449, in <dictcomp>
    f.name: rec[f.name] for f in field.computed_fields
  File "/Users/janno_liivak/development/python/odoo/openerp/models.py", line 5330, in __getitem__
    return self._fields[key].__get__(self, type(self))
  File "/Users/janno_liivak/development/python/odoo/openerp/fields.py", line 679, in __get__
    return record._cache[self]
  File "/Users/janno_liivak/development/python/odoo/openerp/models.py", line 5643, in __getitem__
    value = self._recs.env.cache[field][self._recs.id]
KeyError: 19
>>>

There's an old payment which invoice was credited (or reversed or Refunded ... whatever the term). New invoice was issued and old payment was like an advance payment an it was reconciled together with additional payment for the new (a little bigger amount of money). All was well and good but the date of a new payment was incorrect ... so now I will have to unreconcile the new payment and correct the date.

How to overcome the error? What causes it anyway?