There seems to be a conflict with the add to cart function in VM2 and Akeeba's Admintools (pro-version) protection again cross-site-scripting. When this is enabled in Akeeba, the user can not add anything to the cart. No errors.
A client assumed that this issue was because of my access-extension, but after some debuggin it turns out to be a conflict between VM and Akeeba.
I hope you find a solution, because protection again XSS is a good thing.