The insurance provided for in Article L. 3253-6 does not cover sums contributing to compensation for loss caused by the termination of the employment contract in the context of redundancy for economic reasons, pursuant to a company, establishment or group agreement, a validated collective agreement or a unilateral decision by the employer approved in accordance with Article L. 1233-57-3, where the agreement was concluded and filed or the decision notified less than eighteen months prior to the date of the judgment opening the safeguard, receivership or compulsory liquidation proceedings, or the agreement concluded or the decision notified subsequent to the opening of the safeguard, receivership or compulsory liquidation proceedings.