Hi Jerome,
I found out what's the cause of my problem, not sure if it's a feature or a bug 
I imported the values for the acl on the prices and looking in the database this gave a result like "group1,group2".
When I looked in the backend at the product, the acl's on the price looked fine, just like I would expect.
Unfortunately this did not work in the frontend.
When I changed the values of the acl on the price, just turning them off and on and then saving the product, it worked again in the frontend.
Looking in the database again, the value was changed to ",group1,group2," (added 2 extra comma's).
So I solved my issue, but not sure what the thing is with those 2 extra comma's.
Maybe you know why they are needed or infuencing the behavior on the fron end?
Thanks for your answer,
Nico