Not sure where to post it…
It seems that downloading a file from a Letsencrypt-Secured Domain (https) is not possible because the downloader-application of android do not recognize the letsencrypt certificate.
I was able to reproduce the bug on three devices running all Android 4.2
The HTTPS-Certificate is properly setup and validated with various SSL Tests. When opening the link in Android, the browser recognizes the certificate.
I tried to deploy an .APK through my own Server through HTTPS, didn’t work. So I tried the diawi service (https://www.diawi.com)… same Problem. Both servers have in common that they use HTTPS-Certificates from Letsencrypt. When I try to download the app through normal unsecured HTTP, everthing works.
It seems that the Android Download-App does not recognize SSL-Certificates from letsencrypt.