On the page that you are getting this error on you will need to add in the full request and full response to see what the specific value is that the API does not like.
On this page go to your bindings and expand the PayPal group, put the full request and full response bindings onto the page then save the page and test it. When you go through this time you should see the entire request that was made as well as the full server response. This info should help to clear up what the problem is. You can post back with these details if you are having trouble getting it sorted.