Thank you @zh3ntil for answering, but that didn't work.
Fortunately I could find why that was happening, I don't konw if it's silly or that's the way has to be done.
So here it comes :
The problem was that the subreports resources weren't being found.
@JasperServer -> Report Set up -> Control & Resources
I have to say that when you're adding subreports resources in Jasper Server you must name your resource name and your resource id exactly the same as the physical file is named... So instead of naming your resource 'subreport' you better name it 'subreport.jrxml' including the '.jrxml' extensión.
