This is a bug in the current Appyet Version. There is no solution yet. The only solution is to make a menu entry with the download opening in Webbrowser.
After the failure of the Download (which I was able to fix with a tab opening in external browser), a new problem rised to the wonderful web module: When I upload a file to a web module site and submit the form then net::ERR_FAILED is there! Maybe because @appyet is using UC Browser 11 but...