Hi Audun,
To confirm a few things, when you're editing this product:
- What is the quantity set in the field called: "Qty for Item's Status to Become Out of Stock"?
- What is the field called "Backorders" set to?
...and in System --> Configuration --> Catalog -- Inventory - Stock Options:
- What is the field called "Decrease Stock When Order is Placed" set to?
and lastly, is there any change when you reindex (System-->Index Management) or refresh the cache (System --> Cache Management)
Chances are that when reviewing these settings will help you to solve your issue, but if not, please feel free to confirm settings, and hopefully a member of the community will have an idea for you.
Best of luck!