Seems a bug in one of the modules, probably the sale_shipment_grouping. And I think https://discuss.tryton.org/t/the-value-order-for-field-shipment-cost-method-is-not-valid is also related.
No idea, this is still on 6.8 and 6.8.5 works, but 6.8.6 not? You have to downgrade also the different modules. I would start with downgrading the sale_shipment_cost to 6.8.1 because the sale_shipment_grouping hasn’t been upgraded. If it still doesn’t work, check for the version on http://downloads.tryton.org/6.8/ and downgrade based on the date a new version was released.
Please let us know which module made it working again, so we can check what has changed in that module.