I have been struggling with the same for 2 days now on 1.39h and whatever i try (1.53 and 2.03) still no add product.

I cleaned the whole file back to the bones and it has something to do with the page calling itself.
I think somewhere between 1.38a and 1.39f there has been a change in what is allowed and what not.

Still fighting with it and as someone has a clue?