Sorry but this is definitely NOT fault graves! If you intentionally damage an aircraft or steal something - yes. But this here is something completely different. And by the way these caisse de malady guidelines are not applicable outside luxemburg. What a joke is this? You have a sinusite, you`ll get antibiotiques that won`t allow you to fly for two weeks. Then you go out on your third day of sickness to buy some food in the supermarket and get fired immediately without any pre warning!???
But please let the court decide. Be aware that in Luxembourg court cases take up to five years. Even if you are fired illegally - it takes a looong time. Management knows that and strengthens their strategy of management by fear.