With revision 289 i'm getting the same error:

MERCATO: Login Successful
2014-06-30 15:52:09,542 1674 INFO ? MERCATO: Missing openerp resource for object res.partner with external_id 395. Importing on the fly
2014-06-30 15:52:09,680 1674 INFO ? MERCATO: Calling Method:customer.info,Arguments:[u'395']
2014-06-30 15:52:10,405 1674 INFO ? MERCATO: Query Returned:{'store_id': '0', 'rp_token_created_at': None, 'website_id': '1', 'updated_at': '2014-04-09 07:19:49', 'prefix': None, 'created_in': 'Admin', 'rp_token': None, 'password_hash': '7769a624afa28254c9aa3d0a6b67771f:iSBQ7px7ZErVfZ0ffIKsSxJflMu61EWV', 'taxvat': None, 'increment_id': None, 'default_billing': '395', 'customer_id': '395', 'default_shipping': '395', 'email': '<email address hidden>', 'confirmation': 'FfsbKR', 'firstname': 'Stella', 'middlename': None, 'lastname': 'Oliver', 'suffix': None, 'dob': None, 'gender': None, 'created_at': '2014-04-09 07:19:47', 'disable_auto_group_change': '0', 'group_id': '1'}
2014-06-30 15:52:10,555 1674 WARNING ? openerp.osv.expression: The domain term '('referential_id', '=', [0])' should use the 'in' or 'not in' operator.
2014-06-30 15:52:10,927 1674 INFO ? MERCATO: Created in OpenERP res.partner from External Ref withexternal_id 395 and OpenERP id 22896 successfully
2014-06-30 15:52:11,001 1674 INFO ? MERCATO: Created in OpenERP res.partner.address from External Ref withalternative_keys False and OpenERP id 23707 successfully
2014-06-30 15:52:11,011 1674 INFO ? MERCATO: Updated in OpenERP res.partner.address from External Ref withalternative_keys False and OpenERP id 23707 successfully
2014-06-30 15:52:11,219 1674 ERROR ? MERCATO: Error : No tax id found for the rate 0.434475806452 with the tax include = False
Traceback (most recent call last):
  File "/opt/openerp-ocb/server_launch/openerp/addons/base_external_referentials/decorator.py", line 130, in wrapper
    response = func(self, import_cr, uid, external_session, resource, *args, **kwargs)
  File "/opt/openerp-ocb/server_launch/openerp/addons/base_sale_multichannels/sale.py", line 653, in _record_one_external_resource
    defaults=defaults, mapping=mapping, mapping_id=mapping_id, context=context)
  File "/opt/openerp-ocb/server_launch/openerp/addons/base_external_referentials/external_osv.py", line 727, in _record_one_external_resource
    existing_rec_id = self.oe_create(cr, uid,  external_session, vals, resource, defaults, context=context)
  File "/opt/openerp-ocb/server_launch/openerp/addons/base_sale_multichannels/sale.py", line 759, in oe_create
    vals = self._convert_special_fields(cr, uid, vals, external_session.referential_id.id, context=context)
  File "/opt/openerp-ocb/server_launch/openerp/addons/base_sale_multichannels/sale.py", line 908, in _convert_special_fields
    vals = self._add_order_extra_line(cr, uid, vals, option, context=context)
  File "/opt/openerp-ocb/server_launch/openerp/addons/base_sale_multichannels/sale.py", line 983, in _add_order_extra_line
    raise except_osv(_('Error'), _('No tax id found for the rate %s with the tax include = %s')%(tax_rate, context.get('is_tax_included')))
except_osv: (u'Error', u'No tax id found for the rate 0.434475806452 with the tax include = False')
2014-06-30 15:52:11,230 1674 INFO ? MERCATO: Calling Method:sales_order.done,Arguments:[u'800000001']
2014-06-30 15:52:12,161 1674 INFO ? MERCATO: Query Returned:True