I could fix the issue for real subdomains. Thanks.
I have answer yet for a second and third domain which I want to run on the domain of the webserver:
2016-11-22T10:53:29+01:00 DS1515 synoscgi_SYNO.Core.Certificate.LetsEncrypt_1_create[29502]: certificate.cpp:1359 Failed to create Let’sEncrypt certificate. [1][syno-letsencrypt output is not a json: { “error”: 102, “msg”: "Invalid response from http://xxx.de/.well-known/acme-challenge/jOTbgews_ndzIgnW_F2Nb5UJ25KH7qgAdJPmzfT5jvs: "
So far I have the domeian xxx.de certified,
yyy.de & zzz.de should run on xxx.de. So far I redirected by FRAME redirection on Strato. Does not work for Let’s Encrypt. HTTP redirection does also not work.
Any idea?