Marketpress WPML non-english product pages not found due to bad url


I am trying to replicate the demo setup of

I have a problem with the Spanish product pages, which only work with the (partly) english url version:
The generated spanish (not working) url is:
The working url is:

The problematic part here is /compra/productos/

I have set the Spanish translation for both the store (compra) page and the products (productos) slug. I cannot change the store slug to compra in the Spanish product Presentation Settings as it conflicts with an existing url. I can save the productos slug, but that doesn't seem to have any effect.

What am I missing?