I found the error: It was a mistaken “-” which was not a real “-“, but some strange symbol in the html-footer of my feed by mail-theme. Don’t know how this could be, because I did not change this symbol since last year. The really strange thing was that the “Test” emails always worked fine, except the real delivery.
There are character which cannot be encoded with 3 bytes utf-8 but require 4 bytes utf-8. Not all mysql versions support them (creating big problems with he new smartphone emoji). It could have been your case.
Anyway, just a note: Feed by Mail is a completely separated plugin than Newsletter and I designed it in this way to keep it independent exactly to avoid as much as possible interference between versions :-).