Gerardo,
Have you looked in the logs to see if there are any messages that might help (both the Mendix logs and browser console)? Also, have you checked the permissions on the entity to ensure that logged in users have the proper permissions? Can you reproduce this in Studio Pro?
Answers to some of those questions will help track down the issue.
Mike
this is the page.
this is part of the domain model:
and this is the microflow that is executed when the user clicks on upload button. I added a breakpoint here but it is not being hit, I think the error occurs before.
rThis is the configuration of the upload button