Cannot add same amount of articles twice

[jplw]

I Use J1.7.3/VM 2.0.

I am encountering a problem with the cart when i use Internet Explorer (IE7 and IE9 for example). With FF and Safari the cart works fine.

Problem: I can only once add a specific amount of an article to my cart. When i after that try to add the same amount of the specific article to my cart, the cart doesn't add up.
When i after that add to the cart an amount of the specific article i haven't added before the cart adds up again.

As i mentioned this is only a problem in Internet explorer.

What to do?