Hey. I should had provide the details earlier, sorry about that (and for my english).
If I try to download from iOS (iPad 1 and iPhone 4), error, no file.
If I try to download from a web navigator, it's working.
For information :
I Use the default COPS Web interface.
Access online via IP adress + port 1007 (settings in the synology interface).
The problem is the same over Wi-Fi or 3G.