For me in Odoo V8 the error occured after having accidentially typed in "0" at the tax field of an invoice row and confirmed it by [Enter].
This made the system generate a VAT-Type named "0" without further configuration.
After this accidential happening, the error occurs at every invoice-confirmation.
Even after deleting the wrong VAT-Type "0" the error is still alive.
After a reboot of the Odoo-hosting machine / ubuntu 14.04-64 still no change:
 Odoo Server Error
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 960, in exec_workflow
    return request.session.exec_workflow(model, id, signal)
  File "/opt/odoo/odoo-server/openerp/http.py", line 1101, in exec_workflow
    r = self.proxy('object').exec_workflow(self.db, self.uid, self.password, model, signal, id)
  File "/opt/odoo/odoo-server/openerp/http.py", line 872, in proxy_method
    result = dispatch_rpc(self.service_name, method, args)
  File "/opt/odoo/odoo-server/openerp/http.py", line 114, in dispatch_rpc
    result = dispatch(method, params)
  File "/opt/odoo/odoo-server/openerp/service/model.py", line 37, in dispatch
    res = fn(db, uid, *params)
  File "/opt/odoo/odoo-server/openerp/service/model.py", line 113, in wrapper
    return f(dbname, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/service/model.py", line 183, in exec_workflow
    return exec_workflow_cr(cr, uid, obj, signal, *args)
  File "/opt/odoo/odoo-server/openerp/service/model.py", line 177, in exec_workflow_cr
    return execute_cr(cr, uid, obj, 'signal_workflow', [res_id], signal)[res_id]
  File "/opt/odoo/odoo-server/openerp/service/model.py", line 159, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/opt/odoo/odoo-server/openerp/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/models.py", line 3524, in signal_workflow
    result[res_id] = workflow.trg_validate(uid, self._name, res_id, signal, cr)
  File "/opt/odoo/odoo-server/openerp/workflow/__init__.py", line 85, in trg_validate
    return WorkflowService.new(cr, uid, res_type, res_id).validate(signal)
  File "/opt/odoo/odoo-server/openerp/workflow/service.py", line 91, in validate
    res2 = wi.validate(signal)
  File "/opt/odoo/odoo-server/openerp/workflow/instance.py", line 75, in validate
    wi.process(signal=signal, force_running=force_running, stack=stack)
  File "/opt/odoo/odoo-server/openerp/workflow/workitem.py", line 120, in process
    ok = self._split_test(activity['split_mode'], signal, stack)
  File "/opt/odoo/odoo-server/openerp/workflow/workitem.py", line 248, in _split_test
    self._join_test(t[0], t[1], stack)
  File "/opt/odoo/odoo-server/openerp/workflow/workitem.py", line 257, in _join_test
    WorkflowItem.create(self.session, self.record, activity, inst_id, stack=stack)
  File "/opt/odoo/odoo-server/openerp/workflow/workitem.py", line 95, in create
    workflow_item.process(stack=stack)
  File "/opt/odoo/odoo-server/openerp/workflow/workitem.py", line 116, in process
    if not self._execute(activity, stack):
  File "/opt/odoo/odoo-server/openerp/workflow/workitem.py", line 162, in _execute
    returned_action = self.wkf_expr_execute(activity)
  File "/opt/odoo/odoo-server/openerp/workflow/workitem.py", line 313, in wkf_expr_execute
    return self.wkf_expr_eval_expr(activity['action'])
  File "/opt/odoo/odoo-server/openerp/workflow/workitem.py", line 291, in wkf_expr_eval_expr
    result = eval(line, env, nocopy=True)
  File "/opt/odoo/odoo-server/openerp/tools/safe_eval.py", line 314, in safe_eval
    return eval(c, globals_dict, locals_dict)
  File "", line 1, in <module>
  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 518, in new_api
    result = method(self._model, cr, uid, self.ids, *args, **kwargs)
  File "/opt/odoo/odoo-server/addons/ecoservice_financeinterface/account.py", line 272, in action_move_create
    thisreturn = super(account_invoice, self).action_move_create(cr, uid, ids, *args)
  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 363, in old_api
    result = method(recs, *args, **kwargs)
  File "/opt/odoo/odoo-server/addons/account/account_invoice.py", line 926, in action_move_create
    move.post()
  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 546, in new_api
    result = method(self._model, cr, uid, self.ids, *args, **kwargs)
  File "/opt/odoo/odoo-server/addons/ecoservice_financeinterface/account.py", line 132, in post
    res = super(account_move, self).post(cr, uid, ids, 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/account/account.py", line 1324, in post
    self.write(cr, uid, [move.id], {'name':new_name})
  File "/opt/odoo/odoo-server/openerp/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/addons/account/account.py", line 1365, in write
    result = super(account_move, self).write(cr, uid, ids, vals, c)
  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 363, in old_api
    result = method(recs, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/models.py", line 3741, in write
    self._write(old_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 546, in new_api
    result = method(self._model, cr, uid, self.ids, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/models.py", line 3987, in _write
    recs.recompute()
  File "/opt/odoo/odoo-server/openerp/api.py", line 239, in wrapper
    return new_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/models.py", line 5648, in recompute
    rec._write(values)
  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 546, in new_api
    result = method(self._model, cr, uid, self.ids, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/models.py", line 3852, in _write
    cr.execute(query, params + (sub_ids,))
  File "/opt/odoo/odoo-server/openerp/sql_db.py", line 158, in wrapper
    return f(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/sql_db.py", line 234, in execute
    res = self._obj.execute(query, params)
ValueError: "duplicate key value violates unique constraint "account_invoice_number_uniq"
DETAIL:  Key (number, company_id, journal_id, type)=(VK/2015/0011, 1, 1, out_invoice) already exists.
" while evaluating
u'action_move_create()'
I have the same problem. I am using Versión 7.0-20130510-231240. I tried to cancel and re-validate an invoice which had a mistake and when revalidating I got the "Constraint Error Invoice Number must be unique per Company!" error. After that I am unable to make even a new invoice without getting the error. The next sequence number is ok and I have even tried with a far ahead number so I am sure there is no invoice with that number.
My solution was to review every single accounting type setting by comparing it to a functional company. Fix any errors and then go into the database and manually delete the records without proper invoice numbers. It started working at that point.
I'm having the same problem when I try to validate vendor bills in the accounting module. I'm not sure I understand the way you solve your problem. Could you please provide more details on it?