Hi Javier,
The problem with your UoL is the way the QTI is managed in the callback. The system is ready to manage xml files included in the UoL, instead of zip ones.
So, if you want to add an item, just add the xml file in the resources list and forget about the manifest, which is the other file included in the QTI zip file you sent us.
Thus, only one item is allowed for each resource.
Hope this works,
Cheers,
Luis
