Crispbread should always be stored dry and well-sealed.
Crispbread is a fantastic product. It's healthy, it tastes good and can be stored basically for any length of time without changing the bread's properties. A "Best for Date" on crispbread should be seen more as a quality marker from the bakery that the product retains the same characteristics such as taste, colour and elasticity as it has when it gets produced. A crispbread lasts a long time in ordinary storage, which means that it should be stored dry. So even long after that "Best before date" you can enjoy your crispbread. On crispbread packs, producers usually put this "Best Before Date" at between 6 - 12 months a little depending on the content. There are no ingredients in a traditional crispbread that could make it unfit to eat. What can happen is that crispbread containing seeds or any other seasoning may drop in flavour, but that will occur a long time after The Best before date has passed.
Here you will find knäckebröd that is starting to approach or has passed best before date: Short date
What many people happen to is that the crack bread you have at home becomes soft. The fact that a crispbread becomes soft is quite natural if you have not been keeping it dry and well-sealed. Compare the properties of a crispbread with a plain soft white bread if they were to be there for a long time. A plain white bread contains large amounts of moisture, while a crispbread is dry. What happens then is that nature wants to equalize the difference between the property of bread and their surroundings, which leads to the dry crispbread sucking moisture from the air while soft white bread emits moisture. The result then is that a hard bread becomes soft and soft bread will be hard if they are allowed to be there for a long time and are not stored correctly.
The nice thing about crispbread is that if it has been soft, it's so easy to make the crispbread crunchy again. The only thing you must do is heat the knäckebröd in the oven, a few minutes, at about 100 degrees. After that, you have your crunchy and tasty crispbread again.