they also sent me an email with a photo of their installation to tell me that they were running with a team reduced because of the end of the year holidays and had problems of materials but they worked hard to solve the problems.
so i'm reassured

Is that even an excuse. They know that the market they operate does not take holiday 24/7 all through the year whether holiday or not, it does not stop anything so if they know they employees are on holiday, they should put out a statement to that effect earlier and not wait for someone to suffer for that inefficiencies before stating the reason why or they better emply adhoc staff I am sure a lot of people would forgo the holiday if it would get them some extra cash.