Alright, works now!!
Today I pulled the newest updates from Git and deleted following folders /etc/letsencrypt/[archive | live | renew | csr | keys]
I ran into this error, when I deleted only archive folder: TypeError: coercing to Unicode: need string or buffer, NoneType found
I think the main reason was that I created the text in folder /web/.well-known/amce-challenge with TextEdit tool in DSM. In my former tries, I created the file via Vi (ssh to my Synology-Box) and set file encoding via TextEdit later in DSM. I realized that even when I only had one line in Vi with the temporary LE-string, TextEdit in DSM showed me always an additional 2nd empty line.
Thanks for all your valuable feedback!