Update : Using a html link (instead of an url one) I'm able to use relative path and to access my documents stored in the Content Library.
Now I have another issue. I'm actually storing 400 documents in my Content Library. When I try to add another one, using the QMC, the server proc jump to 100% and the process takes something like 10 min (to upload 10 documents).
Is there a known issue using the Content Library as I do ? A known limitation ? (Qlik sense Server June 2018)